High-grade power unit glectroolcs Simple transmission line ......4 Please mention ELEKTOR...

THE ELECTRONICS MAGAZINE WITH THE PRACTICAL APPROACHUK £1.60 IR £2.35 (incl. VAT) September 1989

glelttor

glectroolcs

Centronics monitorASIC microcontrollersStereo viewerNew generation of analogue switchesHigh-grade power unitSimple transmission line experimentsMicroprocessor tone generatorCommunication receivers front-end filtering

For last delivery telephoneyour order on 01-205 9558using VISA Access Card

BAU.K.D

.11.1Orders welcome fromgovernment depts &educational establishments

TECHNOMATICTechno House 468 Church Lane, London NW9 8TQ.

Tel: 01-205 9558 Fax: 01-205 0190

All prices ex VATPrices are subject tochange without noticePlease add carriage(a) £8.00 (Courier)(b) £3.50(c) £1.50(d) £1.00

rchimedes Computer S) stems

All Archimedes systems are fitted with new RISC OSsystems.

Model BasicA3000 £649310 £899410'1 £1199420 1 £1699440 £1929440'1 £2499PC Emulator £60

Colour£849£1099£1399£1899£2129£2699

Carriage £12 s)stem1.8 computer

For the colour system we offer a choice betweenAcorn Colour Monitor (Philips CM8533) andPhilips CM8833 with RGBAGBi and CompVideo input and stereo sound facility.Every 440 System will be supplied with AcornDevelopers Toolbox and a PC Emulator or FirstWord Plus at no extra cost.

Technoniatic Special Deal-lb get you going on an \ of the aboveArchimedes systems you purchase from us_we will contribute 10% of its cost towardsany additional hardware or softwarepurchase you make from us or to pay forextended finance if you require the facility.

The following NIultiScan \Iunitors can he suppliedat special prices when purchased with a computer:TAXAN 770-1- £425(a) MTS9600 £375(a)

CMI686 16- £1499(a)(for the extra HiRes modes on 400 seriesi

Extended FinanceWe can offer extended finance for 11

months under our special offer detailedabove or for 24r36 month period at 12.75%(approx APR 26%). Please send for details.Technomatic is a licensed credit broker.

Technomatic is an authorisedArchimedes R140 Unix Systems dealer

EXPANSION SYSTEMSRISCOS 1305 3In 4411) £2.9(b)Fitting £101Mb RANI upgrade (410 420) £169103Mb RAM upgrade (410'421)) £49911:0I Mb RAM upgrade A3(X)0 £19911-03.5' Int Drive Upgrade £118(b)( please specif) 3051.310or 4101

External Drive AdaptorExternal 5.25" Drive with psu (40180T)Acorn'_)Mb Upgrade (305 310)Techno 20Mb upgrade kit 141)1)Techno 40Mb upgrade kit (410)Techno Ext HD upgradeAcorn Backplane (2 slots)TechnoLog Backplane (4 slots)Fan for TechnoLogAcorn ROM poduleWe have a large range of expansion cards in stockplease send for details.

Cl£90(b)

£4491a)£199(a)£349(a)

POA£37(c)f39(c)£8(d)

f51(c)

MULTI II0 Podule

rchimedes, SOFTWARE

LANGUAGES & PROG. TOOLSANSI (ASO PASCALFORTRAN 77PROLOG XJLISPTWIN EditorArchimedes AssemblerSoftware Dev. ToolboxRISC BASIC Compiler (SV)Chares Toolkit £32(d) Cares

each £95(c)each E175(b)

£2.7(d)£185(b)£185(b)

£85(c)Toolkit £41(d)

WORD PROCESSORSFirst Word Plus £7910Graphics Writer f27(d)Pipedream £89(c)Pipedream Spellchecker E43(d)

SPREADSHEETSLogistix £95(c) Siemasheet E57(c)

ACCOUNT SYSTEMNfinerva's Suite' each £541d)(Order Processing/Sales Ledger Stock ManagerPurch. Ledger/Nom. Ledger) Requires System Delta plusI tome Accounts £41(d)

EMULATED PACKAGES\ I EW Viewsheet Viev. storeIntemord lntersheetWordwise

each £47(d)each f35(d

£201di

GRAPHICS/ART/CADPro .Artisan £137(b) ArtisanArtisan Support DiscAtelierAuto SketchGamma PlotRender BenderPresenter £24(d) SVARC-PCBSV Solid CAD £41(d) SV Super DumpS% Real Time Solids Modeller

£301d)£17(d)£99( c)£69(d£571c1)£65(d)

0671120£201(1)£75(c)

GAMESHoverbod Missile Control each LIM)Orion,'Freddie's Foll) Jet Fighter each £121diFugitive QuestRise in Crime Overload each £26(chZarch E15(d) Conqueror £231 d IEnthar Seven £29l d) Minotaur £9(d)Clares 3 Games PackThundermonk £12( dl CorruptionPacmaria £16(d) Terramex

£12(d)£20ich£16u11

DATABASESSystem Delta Plus £57(c)System Delta Plus Prog Ref Manual £25(d)Reporter' 'Mailshot ~ each £3.31d ISchool Administrator £125(13)Alphahase 07( di Requires System Delta plus

MISCELLANEOUSPC EmulatorHEARSAY Comms PackageMAYA Terminal EmulatorControl (setup utility)

£79i£551dl£491d1£13(d)

Ex -SERVICE EQUIPMENT

MASTER SERIESAN11315 Master 128 £439(a)The Master package includes: View Word processorand l'iewsheet spreadsheet software and EITHEREducational! Fun software bundle withST4RdataI3ASE package OR A 200K 40T DS singledrive T5200. Please specify your choice.

ADC06 Turbo 65C102 Module £115(b)

Technomatic is a Acorn Authorised EconetReferral Centre. We carry a full range ofEconet Accessories in stock.

DISC DRIVES400K /6-10K per mechanism (all drives)3.25- Double sided (40/80T Sw):T5400 SinglePS400 Single with psuTD800 DualPD800 Dual with psuPD800P Dual with psu plinth mounted3.5 Double SidedTS351 (5.25")P5351 (5.25" Case psu)11)352 DualPD352 Dual with psuCombo Drives 5.25 ± 3.5- & psuPD853PD853P plinth mounted

£80(b)f90ibi

£160(a)£170(a)£185ta)

£69( hi£951b)

£126( al£139(a)

£170tal£190(a)

WINCHESTER DRIVESP3OHD 30Mb 1 lard Dri esOther sizes available

£410(a)

PRINTERSEPSON

1..N04(1:Xs50

£1451a) 1_05(4) £245(a)£285(a) LQS50 £41:151f0

STARLCI" £1491a1 LciliColour £199(a)

LC24-l" £269(a)

NATIONAL PANASONICKXPIllS1 incl master lead £139(a)KXP1124

Integrex Inkjet ColourHitachi 672ND A3 Plotter

£2691a)£515( a)C.1791:11

1-This advertisement can only show an example -1of the range of products stocked byTchnomatic. So send for our latest BBCcatalogue pros iding detailed information andprices on BBC Computer Systems.Peripherals. Software and Books. To obtainyour free copy please complete and return thecoupon below.Technomatic is an authorised BBCComputer and Econet Referral Centreproviding a professional and reliable serviceto the BBC user.

NAME

ADDRESS

Post Codeg 59A low cost card provides: Video Digitiser. Sound

Sampler, a full RS232 interface. 3 RONt skts &Joystick interface. El 07(b)

Our Service department has a number of discdrives. monitors and printers available for sale atreduced prices. Please send for detailed list.

Return to Technomatic Ltd. Techno House.L468 Church Lane. London NW9 8TQ.

_J

CONTENTSSeptember 1989Volume 15Number 170

' Theme of the month inOctober will beSatellite and Cable TV.

Also in the Octoberissue:

Inductance meterSAVE decoderInductance meterOpen systemsA -D conversiontechniquesCD error detectorLogic analyser forAtari STDark -room clockLog-antilog amplifier

Front coverClear compact discs priorto being metallized areseen during production atNumbus Records Ltd,Britain's largest manufac-turer, whose developmentof a new laser -masteringsystem won the Queen'sAward for TechnologicalAchievement in 1987.

Producing a CD masterwith the Nimbus laser -mastering system involvestransferring up to 6,000million bits of information(recorded sound) on to aprepared glass master.This is then transferred tometal stampers by an elec-tro-forming process. Thediscs are pressed fromclear polycarbonate byfully automated injectionmoulding presses.

--ent-1111111L.11 Can Britain maintain its lead in mobile radio?

58 Hayes -compatible V22bis modem

28 Capacitors for RF applications39 A new generation of analogue switches

by Jack Armijos and Tania Chur55 ASIC microcontrollers

by Simon Young62 Practical filter design (8)

by H. Baggot

36 PROJECT: PC as tone generatorby J. Schafer DL7PE

52 PROJECT: Centronics monitorby A. Rigby

12 PROJECT: A high-grade power unitby C. Bolton, BSc

24 PROJECT: Stereo viewerby C.J. Ruissen and A.C. van Houwelingen

44 PROJECT: The digital model train (6)by T. Wigmore

1111=111111tgiece.M.Miffillr48 Resonance meter

by J. Bareford

16 Communication receiver front-end filteringby A.B. Bradshaw

18 PROJECT: Budget FM receiverby J. Bareford

22 Amateur communication receivers: stilla challenge?by A.B. Bradshaw

=111111111111130 PROJECT: Decibel meter

an ELV design38 Simple transmission line experiments

by Roy C. Whitehead, C.Eng., MIEE

Electronics scene 21, 23, 27, 43; Events 47: Readersservices 65

Switchboard 66, 67; Classified ads 74; Buyers guide 74;Index of advertisers 73

A hii-111-crade r0'.% r unit - p. 12

Stereo viewer - p. 24

Decibel meter - p. 30

Centronics monitor - p. 52

ELEKTOR ELECTRONICS SEPTEMBER 1989

4Please mention ELEKTOR ELECTRONICS when contacting advertisers

LEKTROPACKS MAIL ORDERSWITCH MODE POWER SUPPLIES

ASTEC 6441012INPUT SWITCHABLE 116240VVLOUTPUTS. 5Vot 3.75.A. '

12V-ch. 1.5A.121cdt. 0.44

Fced Eurceocket Mars Input wan SM-..Mirter L -.o F.:er.1...asti2 Ort011 SimonSze 175 x 135 x 65 mutt Only £10 each + £2.50p.p. Oaantly prima out re rest

WEIR HSS 100,2INPUT 11&240Vo tsOUTPUTS.5Vot 124. 12Vo3 5A 12Vot 2AFury msed,Brand New. Sze 214 x 117 xEa rim Few only ayaLats.'e at less than VitWe 13008

Only £48.00 + £3.50p.p

COUTANT ML1511D7INPUT 187264Vc/tOUTPUTS. 2 7 Vets 204 Varla3e12Vdts_835 121/0/2.14.538.295 x 131 x 68 min. FU:y casc-o.Brand neer_ Few rady airea ble at less :- a'.5. trade price.

Only £75.00 + £3.50p.p.

TELA/1 LOCAL AREA NETWORKSalable for IBM.XDAT tn.* corroato'4s andAMSTRAD PCs. STARTER PACK CCettlinSSOFTY/ARE + MANUAL + 2 CARDS +LEADS. READY to go for a

----..-. -----.i---2 STATION NETWORK. £179.00

ADDMONAL BOARD + CABLE far e.pansion to 3 - 63 s:a1:-.%-s,- t...-, EC.5-. -351 1 MI: '.seoand. FLO scgtware ted".4-_;cal backup if re.,33Xed. FLO ape:. CT rec-,...es_ £99.00 ea.

UNREGULATED POWER PACKS2402 240V 24V 2A 106x75x60rnm Mot. oso :...se. £3.50PEN902A 240V 91/ 1A 90.65x55mm Mc...tett c:ese £3.50AXIAL FANS120 x 120 x 38r= NIDEC-TORIN-ALPHA V. 23011X4 Brand New £7.00HITACHI VT-TU70E Video Tuner TimerFew criy - brand new £30.00RIBBON CABLE ID way 7i0.2 Cobol coded appLance v.Sktgfor computers buslviess machines etc. 113051 reel £10.00EMI TAPE Recording Tape 1 ei6-124COLNAB SPOOLS USED ONCE ody 1-5 £10.00 each. 6 fo.t E502332TANDERG SERIES 3 Language Laboratory

Co:7-zet.e Lsed system fc, sa'e Dete-s c-. re:4-es:

SECONDHAND[Seam Portable Oscilloscpes INTEGRATED CIRCUITSTwin

Telequipment 131016 DC - 20MHz£275 00

=13305 -- _ _ Irp WAC,5715 _ 15.-m.-=4,-,.. _ 45353n . .67627,23

:

_2509106.363535 1*3315.4303,54 il5c

Te!e quipment D75 DC - 50MHz 1,5-,35,105`. Mol.........541 ',2".;+xce.-....c--.. .534 P8E554 346

..£375.00w' r, 52A dal tanebase

Dual

8563 2571,,!1^2SC971.-MG- X.0......p

imp 31355.511046A43 S005VA .V5_ 1252 11454532-Z384 1336

.41,3.61233774 2506p335 U4 trace p:urg-irts

Telequipment 083 DC - 501.0.tz£450.00

-51.65 Zepaiter.4 __ _ CV0-46. 6534

um._ _ MP 0,31351 Cie. .33.33330=4 . _Ma

Rechargeable Ni-Cad Battery Packs

-a.

STOCK NO.

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2:4n4-05 Z1223!:..

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2'41.r.341.2312232X.332234274$233520113422043214422.5

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DESCRSTION CCMOTOURADON

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for Ir.dustr....1: Ter:- 711M.1. A-.:.Celb packazed and 'assemble -3 t.

Dimensional Requirements. PieaseExActRequiremsrus.

.

VOLTAGE CAPACITY PACEDWI) 1.25L.`.55536 110 4 5

41 ill .,

1.e. 4.9 5 5160 150 7z

_ .6_4.9 36 153 545a5 450 725

-_,6_.4/ 2 4 2 45 153 7.25133 5.45tea 545

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36 261 65135 260 5.5149 251 865

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a -1.1.5 AV.V..A5'., Cs 4120-6557L12610 60 ;pro 35.75risaas; '20 7603 7215

LEKTROPACKS MAIL ORDER141 Bath Road, Hounslow, Middx.TW3 3BT. Tel: 01-572 9737

All prices subject to 15% VAT.Officel orders welcome from

Schools, Govenment and LocalAuthorities.

V5.4

clID. IIIPAMPLIFIERS FROM

DIJMYEffThe UK Distributor for thecomplete ILP Audio Range

111211-111

BIPOLAR AND MOSFET MODULESThe unique range of encapsulated amplifier moduleswith integral heatsink.

.. 3 : £11.33 it V243 12ae.- 5 0:: 1, 0,o 111.3,ni £24.15E11.30 H1-354 1808 8,p,-ta amp ssnmi £37.55f:23.65 i4y3U15041 Sows, amp (65hrr! E37.55

_ 4:- - £18.50 ,705128 61.0 os'e1 Imo £34.95a- a- £18.50 Y05248 1221. Vcsfet am; £42.40

- :-!.4 73 :.- :7- E24.15 5364 1527. ystrt: E66.25

PLATE AMPLIFIERSBipolar and Mosfet modules with the sameelectronics as above amplifiers housed in adifferent extrusion without heatsink.1,1505t.. 227. F-- . , - _ 6.53.15 - 364? 180N em (-loom) £24.13554.124. I 7. . 451'£1420 1,5:3324P 1504 5;P0F53 IMO la 05,,1 124.1550.1224 ::11-0,55,35, 51420 97.051755 EON Mosfa amb. . . E29195W3'2445 : i 1 7 .4 E19 25 1.10521.E.3 540.454-1.e.60 . .[33.05

-22.% .."- 51525 .7:5354.152.7. ,..ps'er . _ E5520

Note: These modules require additional heatsinks

POWER SUPPLIESComprising toroidal transformer and DC board topower the I LP amplifier modules.

-.- ePSU27: ' ..- : 30

£10.35£18330

rli.. E42 rit241:-.2. _552 St05248

£28.15E28.20

P55.141 2 - -5:5-D '4'3324.1032 1-,-,577 £20.45 -7.. -.7 -'' 711 12. E30.25PSU4.7: - 128 E22.60 , ! _ ":: - :45 2.. £31.25PSU422 .'25128 £23.55 ,2_ - 4 £31.25P.Su5': - :44 -iT128 12i U5.15 ,:-,.. -- £33.30P5U522. - - -24 2. E25.15 .75._ - 2 £33.307523522 .' -..,.. .21 E26.15

PRE -AMP and MIXER MODULESThese encapsulated modules are supplied within -line connectors but require potentiometers,switches etc.

HY6 w.th bass sod trels1e 9.25Y56 STere0 pre -amp war. bass and Icet:e £15.00

'4383 Guitar pre amp with special effects _ £18.9586 Vounting board for kiYii £ 1.15866 4taurt3ng board for 54.566 or HYBD E 1.75

POWER SLAVESThese cased amplifiers are supplied assembled andtested in 60 and 120 watt Bipolar or Mosfet versions.0512 60 watt 14ob,1 £75.00 5.1532 6-2 815.55U522 120 wit; 8;colarl4obrn! £83.75 U512 120 ,32:1.7-,!er 5108 35

Pr!cr-s 'code VAT and cart ace 1E3 -Quantity prices available on request

Write or phone for free Data Pack

Jaytee Electronic Services143 Reculver Road, Beltinge. Herne Bay, Kent CT6 6PL

Telephone: (0227) 375254 Fax: 0227 365104


ELEKTOR ELECTRONICS (Publishing)Editor/publisher: Len SeymourTechnical Editor: J BuitingEditorial offices:Down HouseBroomhill RoadLONDON SW18 4JQEnglandTelephone: 01-877 1688 (National)or +44 1877 1688 (International)Telex: 917003 (LPC G)Fax: 01-874 9153 (National)+44 1874 9153 (International)Advertising: PRB Ltd3 Wolseiey TerraceCHELTENHAM GL50 1THTelephone: (0242) 510760Fax: (0242) 226626European offices:Postbus 756190 AB BEEK (L)The NetherlandsTelephone: +31 4490 89444Telex: 56617 (elekt nilFax: +31 4490 70161Managing Director: M.M.J.LandmanOverseas editions:Federal GermanyElektor Verlag GmbHStisterfeld-Strage 255100 AachenEditor: E J A KrempelsauerFranceElektor sariRoute Nationale; Le Seau; B.P. 5359270 BailleulEditors: D R S Meyer;G C P RaedersdorfGreeceElektor EPEKaraiskaki 1416673 Voula - AthensEditor: E XanthoulisIndiaElektor Electronics PVT Ltd.Chhotani Building52 C, Proctor Road, Grant Road (ElBombay 400 007Editor: Surendra iyerNetherlandsElektuur B.V.Peter Treckpoelstraat 2-46191 VK BeekEditor: P E L KersemakersPakistanElectro-shop35 Naseem PlazaLasbella ChawkKarachi 5Editor: lain AhmedPortugalFerreira & Bento Lda.R.D. Estefania, 32-1°1000 LisboaEditor: Jaremias SequeiraSpainIngelek S.A.Plaza Republica Ecuador2-28016 MadridSwedenElectronic Press ABBox 550514105 HuddingeEditor. Bill CedrumDistribution:SEYMOUR1270 London RoadLONDON SW16 4DH.Written and composed on Appleand IBM corporate publishingsystems by Elektor Electronics.Prin ad in the Netherlands byNDB Zoeterwoude.Copyright - 1989 Elektuur B.V.

ABC

CAN BirITAIN MAINTAINLEAD IN MOI -LE Is\ AD -0?

One of the great success stories in the United Kingdom electronics industry overthe past few years has been, and still is, mobile radio. Britain's world lead in thisfield has helped to push the number of two-way mobile radios in the UK to wellover a million. Cellular radios, although they became available only in 1985,already account for over half that total. And demand keeps growing.

But while demand continues to grow, there are increasing shortages of skilledengineers and technicians to produce, install and service the equipment.According to the Federation of Communication Services (Fcs), the mobile radiomarket is growing at well over 10 per cent per year, while a survey of its mem-bers shows that 90 per cent of them need more technical staff. One industrysource estimates that 6,000 more specialist staff will be needed by 1995.

There are several initiatives that mobile radio firms should make the most of todemonstrate that mobile radio offers excellent career prospects. These include theEnterprise and Education Initiative, which aims to strengthen the partnershipbetween business and education by offering young persons the opportunity ofgaining work experience in both manufacturing and service industries. and teach-ers the chance to experience business first hand. Another effective way of draw-ing school-leavers' attention to the radio industry is through the Young RadioAmateur of the Year Award. This is aimed at anyone under 18 who is keen on DIYradioconstruction or operation, uses radio for a community service, or is involvedin amateur radio in some other way, for instance, a school science project.

One of the main problems in mobile radio is the lack of nationally recognizedqualifications for technicians. Trainees are often attracted to other sectors wherestructured training exists. The mobile radio industry itself faces difficulties whenrecruiting technicians of indeterminate abilities. Consequently. mobile radio userssuffer because of the varying quality of service they receive.

These problems led the Department of Trade and Industry, the Mobile RadioUsers Association (NtRuA). the Federation of Communications Services and theElectronics Engineering Association (EEA) to start the RadiocommunicationsQuality Assurance Scheme. For a company to maintain certification with thescheme, technicians must be properly trained and qualified. Recognizing this, theDTI and the MRLA earlier this year launched a joint initiative. This resulted in theMobile Radio Training Committee (mwrc), whose aim is the identification of themobile radio community's education and training needs.

The dialogue between academics and industry is important. Academics haveexpressed the view that business people should participate in planning coursesand helping to provide on-the-job experience. Educators and trainers should beup -dated by working with businesses, having contact with senior engineers andexperiencing the use of modern equipment.

The activities of the DTI. the MRUA, the FCS and the mwrc are drawing attention tothe importance and growth of mobile radio. The United Kingdom currently has aleading role, but this position is threatened by the shortage of skilled personnel.

The Government is doing much to highlight the career opportunities and alleviatethe problems, but the onus must be on business to form a partnership witheducation. Packages are required that will attract the people needed, in thenumbers required. and provide them with the necessary skills.


A HIGH-GRADE POWER UNITC. Bolton BSc

These supplies were developed to power experimental electronicequipment including small RF oscillators and amplifiers. There are

two versions: a single -channel unit and a dual -channel unit in whichthe channels may be used independently or in series to give

well-balanced positive and negative rails.

Various circuits may be used to produce avariable, regulated output voltage:

Chopper circuitsIn these, the current is chopped into pul-ses which are fed to an energy storagedevice to give an output voltage. This typeof circuit was discounted for the presentdesign since the switching involved pro-duces RF energy which readily interfereswith other equipment.

Shunt regulatorsThese circuits produce a larger currentthan is required, and shunt the unwantedpart away. The shunt regulator is particu-larly wasteful when the required currentis much smaller than the available cur-rent, as is frequently the case in ex-perimental work.

Series regulatorsThese are in essence series resistors thatcan be varied to maintain a constant out-put voltage. Their inefficiency is highest

Table 1.

High-grade power supply

Measured performance of each channel:

Output voltage:Output current:Current limit:Output resistance:

0-25 V fully variable1.5 A max.

50 mA-1.5 A2 mil

Output change for 10% mainschange:Total ripple plus noise

Dual -channel unit only:

<1 mV<1 mV

Output balance: within 10 mV, retainedunder current limit conditions

at low output voltage settings and highload currents. Since the ability to powersuch a load was considered to be the least

frequent requirement, this type of circuitwas chosen for the design.

The measured performance of the units issummarized in Table I.

Single -channel unitThe circuit diagram of the single -channelpower unit is given in Fig. 1. The outputcurrent is produced by Tr2, Bri and C2. Theoutput voltage is controlled by a seriesregulator in which Ts, T4 and T7 are theactive elements. In effect, these transistorsform a multi -stage emitter follower that isdriven by opamp ICI. The current gainand the use of Darlington -type powertransistors for T4 and T7 ensure a smallcurrent demand on ICi. Transistors T4 andT7 are connected in parallel with smallemitter resistors to distribute heat dissipa-tion.

The output voltage of ICI is deter-mined initially by a reference voltage ap-plied to its non -inverting input. The

BC 212

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Fig. 1. Circuit diagram of the single -channel version of the power supply.


A liIGH-GR.N DE PONN ER UNIT

Fig. 2. Suggested construction of the high-grade power unit.

inverting input is a fixed fraction of theoutput voltage supplied by the unit. Thehigh gain and differential operation of ICIenable the device to vary its output volt-age such that the voltage difference be-tween its inputs is almost zero.

The reference voltage for ICI is derivedfrom constant -current source T3 and zenerdiode D4. Components R3 and C4 form asimple noise filter. Zener diode D5 pro-duces a small off -set voltage to enable theoutput voltage to go down to zero.

Current limiting on the basis of voltagefeedback is achieved by Rio, 1C2, T9, TIOand Tb. The current flow through Ri pro-duces a voltage across the resistor. Part ofthis voltage is selected by potentialdivider P2 and Rio, amplified by IC: andapplied to a trigger circuit around Ts andT. Normally TQ is off, but it is turned onwhen the output of IC: rises sufficientlybecause of a higher load current. Thiscauses LED 11 to light, indicating currentlimiting activity, and Tio to be switchedon. Transistors T-, Titi and Tb now act asan amplifier to draw current through Rs,which in turn reduces the reference volt-age to ICI and, consequently, the outputvoltage.

Power to operate the reference sourceand associated circuitry is obtained inde-pendently of the output supply from Tri,Bri and CI, together with stabilizing cir-cuit Ti and T2. Loading on this supply isELEKTOR ELECTRONICS SEPTEMBER 1989

constant until current limiting occurs. Theregulator for this supply thus acts onlyagainst fluctuations on the mains, whichrarely reach 10%. This enables a steadyreference to be obtained fairly simply.

Practical pointsComponent layout is not critical, but at-tention must be paid to a number ofpoints. The can of C2 must be well sleevedto keep it insulated from the chassis. Thewires carrying the output voltage must berouted such that they do not form loopsenclosing other components (this is mostlikely to happen on the front panel). On asimilar note, the wires carrying the outputcurrent must be thick enough to preventundue heating, and the wire connectionsat the output terminals must be madeexactly as shown in the circuit diagram.

As to cooling, T4 and T7 must bemounted on a heat -sink with a thermalspecification not exceeding 0.5 K/W. Re-member to insulate these transistors elec-trically from the heat -sink. Transistor T2requires only a small heat -sink.

Fuses F2 and F3 are intended to protectthe rectifier bridges and the transformersagainst failure of the smoothing capacitor.They consist of short lengths of 40 SWGcopper wire: F2 between vero-pins on thecircuit board, and F:: between tags on ashort length of tag strip, which can be

rear vanes and heal sins

mounted anywhere convenient to thetransformer leads.

Any type of non -steel cabinet may beused to house the power supply. Steelmay be used provided the main transfor-mer has sufficiently small magnetic leak-age to avoid magnetizing the steel nearthe leads to the inputs of IC:.

Constructional details of a cabinet thatmay be made from aluminium are givenin Fig. 2. No dimensions are given sincethese will depend on the componentsused for Tr2, C2 and the heat -sink. TheI --section is extruded aluminium, which isavailable from many DIY suppliers.

Setting upThe setting up procedure is concerned en-tirely with the current limit facility.1. With the unit switched off, set the out-put voltage control, Pi, for zero volts, thecurrent limit control, P2, for maximumcurrent (maximum resistance), and Ps tozero resistance.2. Connect a resistor of about 10 Si, ca-pable of carrying 1.5 A, across the outputterminals (a length of electric fire spiralhas been found useful).3. Switch on the unit and raise the outputuntil a current of 1.5 A flows.4. Adjust P4 so that the current limit warn-ing light, Di3, is just on.5. Increase the resistance of Ps until the

14 GENERAL INTEREST

Fig. 3. Circuit diagram of the dual -channel PSU.


A HIGH-GRADE POWER UNIT

current drops by between 50 and 100 mAas indicated on an ammeter.6. Reduce the output voltage to zero andcheck that the warning light goes out.7. Raise the output voltage and check thatthe warning lamp comes on at 1.5 A.8. Try to raise the current by raising theoutput voltage or reducing the load resis-tor, and check that there is little rise inoutput current.9. Choose other settings of the currentlimit control, and check that limiting oc-curs at lower currents. The lower limitshould be between 30 and 30 mA.10. If at any time the current limit indica-tor lights, but at less than full brightness,the limit circuit oscillates because P3 hasbeen advanced too far and should be re-adjusted. This is best done by reducing itsvalue to zero and repeating operations 3,4 and 5_

The current limit control can be calibratedby setting it to maximum, adjusting theoutput current to a value required as acalibration point, and then adjusting thelimit control until limiting just occurs asindicated by the lamp coming on.

Notes on the useThe output of the single -channel unit isfloating so that either side, or none, maybe grounded. The high degree of regula-tion is available only direct at the outputterminals of the supply: bear in mind thatsix inches of ordinary connecting wirehave a higher resistance than the outputresistance of the unit.

Under near short-circuit conditions,the current limit may produce a low-leveloscillation on the output voltage. This isdependent on the reactance of the load,and is unlikely to be of any consequencesince the supply is not normally used as aconstant -current source.

Dual -channel unitIn the dual -channel unit, channel 1 is es-sentially the same as the single -channelunit. The modifications are the addition ofa fine voltage control, Ps, a second currentlimit amplifier, IC1, which is operationalonly in the balanced mode, and a dis-charge circuit, Tit, Tiz and T13, which dis-charges CO when the voltage setting isreduced, enabling the output to follow thesetting closely.

Channel 2 is similar to channel 1 except

Fig. 4. Auxiliary circuit for adjusting thePSU.ELEKTOR ELECTRONICS SEPTEMBER 1989

that it is complementary. For ease of fol-lowing, the circuit components with anidentical function in channel 1 and thesingle -channel version are given the samereference numbers. Likewise, compo-nents in channel 2 serving the same func-tion as those in channel 1 are given thesame numbers with prefix '9'. Thus, ICI ofchannel 1 becomes I041 of channel 2. Thecomplete circuit diagram of the dual -channel power supply is given in Fig. 3.

The discharge circuitAs the voltage setting is reduced, the out-put of ICI falls, and will fall below theoutput terminal voltage unless C. is dis-charged. The output voltage of ICi is de-veloped across via emitter followers Isand Tn. Diode Dio produces a small volt-age to compensate for additional base -emitter drop in Darlington transistors T4and 17. If the voltage across R33 is lowerthan that across CO, 112 is turned on. Thisin turn switches TIS on, which dischargesCo until the voltage across it is almostequal to that across R33 when Tia is turnedoff. Components D12, DIS and I236 limit thebase current in TIS to a safe level. DiodeDii prevents the base of TI2 being drivendangerously positive if the voltage settingis raised suddenly.

Balanced output modeIn the balanced output mode, the oper-ation of channel 1 is unchanged. In chan-nel 2, the reference voltage is obtainedfrom the channel 1 reference via the'times -1' amplifier, IC;. This reference iscompared with 3/4 of the voltage betweenthe positive and negative rails producedby potential divider Rig -R;(1.

If the current in channel 1 exceeds theset limit, IC2 causes the limit circuit tooperate. If the current in channel 2 ex-ceeds the limit setting, the output from IC;causes the limiter in channel 1 to operate.Since both channels use the channel 1 ref-erence, they are limited equally in bothcases. Hence, balance is maintained undercurrent limiting conditions. Diodes Dsand D4 prevent competition for limitingbetween the channels. The current limitsettings of the two channels are inde-pendent.

Switching between modes of operationis accomplished by S2, which is a waferswitch made up of two 6 -pole, 2 -way wa-fers. Relay Ref is operated by Si to switchthe output current.

Current for the relay coil is obtainedfrom the channel 2 AC supply via D14 andCis. This supply also feeds D-, the 'power -on' indicator.

Setting up procedureWith the unit set for independent channeloperation, set the current limit circuits asdescribed for the single -channel unit. UsePs and P4 for channel 1, and P93 and 1-1,,t forchannel 2.

To adjust the balance, either a digital

voltmeter capable of resolving millivoltsat 25 V and below, or the auxiliary testcircuit shown in Fig. 4, is required. ThePSU must be switched on at least fiveminutes before the balance is adjusted.

Turn S2 to balanced operation. Setchannel 1 to about 10 V and adjust P7 sothat channel 2, now the negative rail, alsosupplies 10 V.

Setting up with a DVM11. Set the output voltage to about 19 Vwith the aid of the channel 1 control.12. Connect the digital meter to the + and± terminals. Note the reading.13. Connect the digital meter to the ± and- terminals. Adjust P7 to give the readingobtained in step 12.14. Reconnect the digital meter to the +and ± terminals. Reduce the output volt-age to 1.5 V and note the exact reading.15. Connect the digital meter to the ± and- terminals. Adjust Ps to give the readingobtained in step 14.

Setting up with the auxiliary test circuit11. With the 18 V battery in the test circuitand the multimeter on the 25 V range, con-nect point X to the + terminal, and point Yto the ± terminal. Adjust the output volt-age so that the multimeter reads zero.Change to 100 my and adjust the outputvoltage to give a multimeter reading of50 mV.12. Set the multimeter to the 5 V range.Connect X to the ± terminal, and Y to the- terminal. Adjust P7 until the multimeterreads zero. Change the multimeter rangeto 100 mV and adjust P7 to give a readingof 50 mV.

13. Disconnect the test circuit from theunit. Replace the 18 V battery by the 1.5 Vcell in the test circuit. Reduce the outputto about 2 V and set the multimeter to the5 V range. Connect X to the + terminal,and Y to the ± terminal. Adjust the unituntil the multimeter reads zero. Changethe multimeter range to 100 mV and ad-just the output voltage to give a readingof 50 mV.

14. Set the multimeter to the 5 V range.Connect X to the ± terminal, and Y to the- terminal. Adjust Ps so that themultimeter reads zero. Change themultimeter range to 100 mV and adjust Psto give a reading of 50 mV.

Further settings common to both meth-ods:16. Repeat steps 11 to 15.17. Repeat steps 11, 12 and 13. If any ad-justment of P7 is required, steps 14 and 15must be repeated, followed by steps 11,12, and 13 and so on until no further ad-justment is required.18. Connect the 10 Q resistor used for set-ting the current limit to the ± and - termi-nals. Set the channel 2 current limitcontrol for maximum current, and adjustP. so that the channel 1 limit warninglamp just comes on when the current inchannel 2 (the negative rail) reaches 1.3 A.

16

COMMUNICATION RECEIVERFRONT-END FILTERING

by A. B. Bradshaw

In communication receivers. whetherintended for general coverage or for ama-teur bands only. front-end design haschanged considerably over the years. Withthe use of higher intermediate frequencies(w) and the availability of high -frequency(1F) crystal filters, we no longer see themultiple banks of tuned circuits and multi -gang capacitors.

Unfortunately, for most new develop-ments there is a price to pay: the reductionin pre -mixer selectivity means that anyamplifier preceding the mixer must offersuperlative performance in terms of inter -modulation distortion and cross modula-tion. If it does not, the user may get theimpression that the receiver is full of sig-nals. The old saying that "The wider thewindow's open. the more muck blows in"is very apt here.

It was with these thoughts in mind thatthe writer has designed some general-pur-pose front-end filters for the amateurbands. If you need more protection upfront when Joe Blogi.-.,s just down the roadfires up his 400 watts of sideband, thesefilters should help you to listen on the nextadjacent band up or down. You may wishto incorporate them in your next receiver.

What kind of filter?Frequency filters fall into four categories:low-pass (t_P). high-pass Ow). band-pass(BP). and band -stop.

The design of a band-pass filter for rel-atively small bandwidths is not too diffi-cult, but the difficulty increases exponen-tially with increasing bandwidth!

Band-pass and band -stop filters may beconstructed from a mix of low-pass andhigh-pass sections. In BP filters, these sec-tions are in series: in band -stop filters theyare in parallel. The band -stop filter so con-structed is not often seen in print, but is.nevertheless, a thoroughly practicaldesign. It is, of course, a pity that the LP

and HP sections can be used only for theconstruction of a band-pass or a band -stopfilter. but not for both simultaneously!

In modern filter design. a number ofapproximations to the ideal brickwallresponse have become popular. The low-pass responses of these are shown inFig. 1. Their high-pass response isobtained by network transformation.

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Unequally spaced ripples in pass band. Attenuation(minimum) is defined in stop band. Ripples in stophand. Nfore difficult to set up than Butterworth. Verygood pass -stop band transition.

...

Fig. 1. Frequency response characteristics of the ideal low-pass filter and three approximations.

The operating impedance, band edge.attenuation in the stop band, pass -handripple and component values arc all de-rived from a low-pass section normalizedfor a frequency of 1 radian and an impe-dance of 1 D.

The shape of the response. whichdetermines the complexity (length) of thefinished filter. is decided with the aid ofdesign tables. There is usually a trade-offbetween the ratio of the band -edge fre-quency to the design attenuation frequencyand the stop -band attenuation. This meansthat the 'squarer' the response of a givenfilter is. the lower will be the stop -bandattenuation.

In the construction of a BP filter. theband edge of the t_P section becomes theupper profile and that of the 11P section.the lower profile. In effect, the two res-ponses cross over each other.

In the designs illustrated in this article.the elliptic function approximation is used.With this. the minimum stop -band attenua-tion remains at its design figure. in con-trast to Butterworth or Chebishev func-tions where it increases the further the fre-quency is away from the band edge. Thisis. however. a small price to pay for theexcellent transition band selectivity of thistype of filter.

The filters discussed here are designed

for a stop -band attenuation of 40 dB or80 dB to meet both light and stringentrequirements. Also. they are designed foran input and output impedance of 50 O.Although intended primarily for receiverapplications, they may, of course. be usedin transmitters, in which case the compo-nent ratings MUST BE UPGRADED!

ComponentsIdeally. the filters should be constructedon a printed -circuit board. but this is notessential.

Capacitors should be low -loss types.They should be connected in parallel toget their tolerance within l'. although sil-ver -mica capacitors with I tolerance arereadily available.

The Q of the inductors should be ashigh as can be obtained. However. as thefilter impedance is 50 Q. the values ofinductance are low, so the coils can bewound manually quite easily. Take care toprevent inductive coupling between sec-tions.

When setting up the filter. trim thecoils to their correct value by checking thestop -band nulls on an oscilloscope (oranalyser if you are that lucky!). Check allfrequencies with a suitable counter.


RADIO & TELEVISION

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2.875 MHz and 4.624 MHz. The frequencies of infinite attenuation are at: LF 2.202 MHz and 6.038 MHz. The frequencies of infinite attenuation ar at: LF2.8 MHz and 1.994 MHz; HF 4.75 MHz and 6.669 MHz. 1.053 MHz, 1.802 MHz and 2.159 MHz; HF 6.158 MHz, 7.378 MHz and

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11.50 MHz and 17.28 MHz. The frequencies of infinite attenuation are at: LF at 8.810 MHz and 22.564 MHz. The frequencies of infinite attenuation are at:

7.977 MHz and 11.2 MHz; HF 17.75 MHz and 24.92 MHz. LF 4.215 MHz, 7.209 MHz and 8.638 MHz; HF 23.01 MHz, 27.57 MHz and47.156 MHz.

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99.625 MHz.


ri,,DGET FM RECEIVERJ.BEtreford

Most receivers for 30 MHz and up contain a fair number ofsemiconductors. Although super -regenerative receivers have been

built with few transistors, their performance is generally rather poor.The present four -transistor receiver is of the super -heterodyne type,and demonstrates the feasibility of building a sensitive FM receiver

from a minimum number of components.

Although the super -heterodyne receiverrequires relatively many sub -circuits, it isprobably the best choice when a receiverfor the VHF and UHF ranges is to be built.

The first building block of the'superhet' is the input stage that filtersand raises the aerial signal to a level suit-able for the second block, the mixer. Thefilters are required to ensure sufficientimage rejection. The third block is thelocal oscillator. This circuit is tuneableand drives the other input of the mixer.The output of the mixer is connected to theinput of the intermediate frequency (IF)amplifier that provides the required selec-tivity. The IF amplifier is generally dimen-sioned for relatively high gain and abandwidth not greater than strictly re-quired. The last building block of thesuperhet is, in principle, the demodulator.

In most receivers of the super -hetero-

dyne type, each of the above blocks usesat least one transistor - the IF amplifierusually requires several. Less conven-tional receiver design, however, allowsthe number of transistors to be reduceddrastically.

Active down, passive upThe circuit diagram of Fig. 1 shows thatthe number of active components has beenkept to a minimum, while relatively manypassive parts are used. Only three transis-tors take care of all the high -frequencyfunctions, and one is used as an audioamplifier.

Transistor Ti and associated parts formthe RF input amplifier that raises theaerial signal. A grounded -base circuit isused to achieve acceptable matching ofthe input cable impedance. Since it is fair-

ly heavily damped, tuned circuit Li-Ci hasa relatively low loaded Q (quality) factor.The tuned circuit can not, therefore, en-sure the receiver's selectivity, but this isnot a problem because the intermediatefrequency is low at 200 kHz, making ade-quate image rejection impossible anyway.The upshot of this is that two frequencies,spaced at 400 kHz, arc actually received atthe same time. In practice, this means thatall stations simply occur twice when thereceiver is tuned across the VHF FMbroadcast band.

The use of a pnp transistor in the RFinput stage allows the amplified signal atthe collector to be coupled direct to gate -I(g1) of MOSFET T2.

The self -oscillating mixer is, admittedly,infamous with many high -frequency en-thusiasts. The design presented here,however, suffers none of the disadvant-ages like poor mixing characteristics,noise, and 'pulling' of the input stage, as-sociated with traditional bipolar transis-tor designs. MOSFET T2 is basically usedin a standard mixer configuration becausethe oscillator signal is available at gate -2,and the received signal at gate -I. Uncon-ventionally, however, the mixer has afeedback network between the source andgate -2. This is achieved with parts L5-05and Di that cause the mixer to oscillate.Varicap diode Di has no effect on the re-ceiver tuning (this is taken care of by Cs),but serves to rectify the RF voltage acrossL3. The rectified voltage makes gate -2 ofthe MOSFET a little positive with respectto ground. The internal gate -source capa-citance serves to buffer this d.c. level,which raises the transconductance of theMOSFET for the signal applied to gate -I,but at the same time lowers the transcon-ductance for the signal applied to gate -2.The combined regulation causes the over-all amplification in the oscillator to stabi-lize at a certain level, and results in arelatively clean output signal.

High -frequency components in themixer output signal are suppressed by Cband series network Cs -Ls, so that a cleansignal is applied to the IF amplifier, T3.Choke L4 prevents the IF signal being


BUDGET FM RECEIVER 19

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Fig. 1. Circuit diagram of the budget FM receiver. The dashed parts are for the optionaldemodulator to be connected to the r.iPx output..

short-circuited by the decoupled positivesupply rail.

One -transistor IF amplifierThe fairly high conversion gain ofmixer /oscillator T2 allows a single transis-tor to be used as the IF amplifier. A dual -gate MOSFET is used as in themixer/oscillator, but T3 is a Type BF982rather than a BF981 because a highertransconductance is required. The func-tion of Lo is similar to that of L4.

The IF amplifier is not tuned - theonly IF filter in the receiver, L7 -C14, is lo-cated behind the limiter, D2 -D3.

DiscriminatorApart from its function as the only IF filterin the FM receiver, tuned circuit L7 -C14acts as an FM demodulator. When theseries combination resonates at about200 kHz, the voltage across the inductor,L7, has the same amplitude as that acrossC14, but a phase difference of 180'. Forfrequencies above the resonance fre-quency, the reactance of the inductor isgreater than that of the capacitor. Thismeans that the voltage on the inductor isgreater than that across the capacitor. Forfrequencies below the resonance fre-quency, the capacitor voltage exceeds theinductor voltage, although the phase dif-ference is still 180'.

Since the IF signal is frequency modu-lated, the voltages across L7 and C14 can berectified to recover the modulation signal.Diode D4 rectifies the voltage across C14,and D, the voltage across L7. Since theELEKTOR ELECTRONICS SEPTEMBER 1989

cathodes are interconnected, the total di-rect voltage with respect to ground equalsthe difference between the individualdiode voltages. This difference is, ofcourse, 0 V at the resonance frequency ofL7 -C14, while it is positive or negative de-pending on the instantaneous signal fre-quency, which is a function of themodulation signal because frequency -modulated (FM) signals are received.

The currents supplied by D4 and D3during their conductive half -cycles areused to charge C13 and CI5 respectively.These capacitors are slowly discharged byparallel resistors R12 and RI1.

Headphone amplifierThe demodulated audio signal is appliedto low-pass filter R13 -C1., which attenuatesfrequencies above 5 kHz and so forms abasic de -emphasis.

The filtered AF signal is fed to head-phone amplifier T4. This FET provides suf-ficient power only if headphones with animpedance greater than 1 1St are con-nected. For higher output volume and/ora lower output impedance, use an LM386-based AF amplifier.

AFC and stereoAutomatic frequency control (AFC) isfairly simple to implement in the receiverby adding the parts connected by thedashed lines in the circuit diagram. Theadded parts create a feedback path be-tween the audio signal and tuning ele-ment Do. The result is automaticfrequency correction, so that the receiver

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remains tuned to a station in spite of fre-quency drift of the local oscillator owingto temperature changes. An additionalbenefit of the feedback circuit is an im-provement in the linearity of the audiosignal, which results in a better high -fre-quency response.

The high boost achieved by the addi-tion of the AFC parts allows a multiplex(MPX) demodulator to be connected tooutput NIPX to enable stereo reception. Ina stereo MPX signal, the L-R differencesignal is modulated on to a subcarrier at38 kHz. Without the AFC circuit, this sig-nal would be difficult to recover from thenormal AF output because of the roll -offat 5 kHz.

The use of a stereo FM demodulator,which is not discussed here, also requiresCi and RI3 to be changed to the valuesindicated in the circuit diagram.

ConstructionThe printed circuit board shown in Fig. 2is double -sided, but not through -plated.The largely unetched component sidefunctions as a ground plane that preventsparasitic coupling between componentsand PCB tracks.

Drill the PCB and sort the componentsbefore fitting them. A large number ofparts is mounted vertically. As the compo-nent density is fairly high, these partsmust be installed carefully, and solderedaccurately to prevent short-circuits.Solder component terminals at both PCBsides if the solder spot at the track side isnot connected to a track or other spot(s).

The mounting of SMD capacitor C4 at

20 RADIO AND TELEVISION

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= 100kRio = 221)R13 = 220k (101C)R14' = 10k

= 'IMO logarithmic potentiometerP2 = 50052 preset H

Capacitors:Ci = 20p foil trimmer greenC2 = 10n ceramicC3 = 39pC4 = 1n0 SMDCs = 14p tuning capacitorG6= 22pC7:C9;C17;G21.:C23.= 100n ceramicCa = 47pCio;C14 = 120pCif = 220nC12 = 47nCis;Cis = 390pCis = 150p (270131Cia;Cis = 47u; 16 VCa' = 270pC22. = 4p7

Semiconductors:Di ;D6* = BB4O5GD2:D3 = 1N4148D4;D5 = BAT85TI = BF324T2 = BF981T3 = BF982T4 = BF25613

Inductors:Li;L2;1_3;Ls = home-made inductor; windingdetails are given in the text. Parts required:1 mm dia. silver-plated wire and 2 ferritebeads (3mm).L4 = 100 mH radial choke with ferrite encap-sulation, e.g., Toko 181LY-104 (Cirkit).Ls = 10 mH radial choke with ferrite encap-sulation. e.g., Toko 181LY-103 (Cirkit).17 = 4mH7 axial choke, e.g. Siemens878108 (Cirkit stock no. 35-71475).Le = 33 mH radial choke with ferrite encap-sulation, e.g.. Toko 181LY-333 (Cirkit).

Miscellaneous:PCB Type 890118 (not available throughthe Readers Services).High -impedance headphones or earpiece.Co -axial aerial socket: e.g., BNC or SO -239.Meta! enclosure.Mains adapter socket.

AFC option

Fig. 2. Double -sided PCB for the FM receiver. Fit screening plates over the dashed lines.


ENBUDGET FM RECEIVER

Fig. 3. SMD capacitor C4 is soldered at thetrack side of the board, between the spotsprovided for R3.

the track side of the PCB is shown in thephotograph of Fig. 3.

The source terminal of 12 is connecteddirect, via a short wire, to the tap on L3.The other terminals of the MOSFET areinserted in the appropriate PCB holes.

The tuning capacitor used in the proto-type was a 14pF single -gang type fromSchwaiger. Other types with a differentmechanical construction are also suitable,provided the capacitance range is about5pF to 14pF. The Schwaiger capacitormust be modified before it can bemounted on to the PCB: cut off the termi-nals, and solder a single wire at the otherside of the capacitor body. Solder quicklyto prevent the stator package being dis-lodged and causing short-circuits with therotor blades. Scratch off the coating fromthe sides of the capacitor body so that thedevice can be soldered on to the PCB.

Install 15 mm high tin plate or brassscreens on to the PCB. The placement isindicated by the dashed lines on the com-ponent overlay.

The receiver must be powered from alow -noise, regulated 12V source. A simplemains adapter will generally be unsuit-able unless additional regulation is in-stalled. In most cases, a Type 7812regulator with associated decoupling ca-pacitors is adequate for this purpose, butmake sure that the unregulated input volt-age is between 15 and about 20 V.

InductorsWind four inductors as detailed below.

Inductor Li: close -wind 8 turns of 1 mmdia. (SWG20) silver-plated wire on to an8 mm drill. Make sure that the ends of theinductor are in the same plane (seeFig. 4.). Then bend the ends at rightangles, keeping them central under theturns as indicated by the dashed lines inFig. 4. Now space the turns evenly so thatthe terminal wires align with the holes inthe PCB.Inductor L3: this is made as Li, but with 6turns rather than 8. The location of the tapis shown in Fig. 4.Inductor LE wind 2 turns of 0.2 mm dia.(SWG36) enamelled copper xvire througha 3 mm long ferrite bead (see Fig. 5a).Inductor Ls: as L2, but with 4 turns (seeFig. 5b).

The remaining inductors in the receiverare ready-made chokes. Suggested typesare given in the Parts List.

AlignmentApply 12 V d.c. to the completed receiver,connect an aerial (75 SI) and headphones.Set the tuning capacitor to maximum ca-pacitance and compress or stretch L3 untilthe local oscillator operates at 88 MHz.Check this with a frequency meter or an-other FM receiver. if necessary, reduce thenumber of turns of L3.

Fig. 4. Winding details of inductors Li and

Fig. 5.and Ls.

Winding details of inductors L2

Tune to a weak transmission and peakCi for maximum output volume. Finally,adjust preset P2 for an acceptable com-promise between distortion and outputvolume.

British Gyros Will Line UpNew Intelsat Comsats

Gyros from Ferranti will ensure the initialline up and correct attitude on station ofthe new series of American Intelsat -7communications satellites.

Similar Ferranti equipment has alreadybeen used successfully in satellites such asIRAS. Exosat. X4 (Miranda) and for theSpacelab Instrument Pointing System(iPs). Moreover, it has been provided forthe Rosat X-ray Astronomical Satelliteand the Olympus communications satellite

Analogue Touch Sensors

Analogue touch sensors that use surfacechemistry and surface electronics for inputvia visual display units (vms) have been

developed by John McGavin & Co.The sensors consist of two conductive

coatings applied to a substrate of polyesteror polycarbonate.The faces are separatedby clear dielectric spacer dots and arebrought into electrical contact only whenactuated by the pressure of a finger.

Computer Graphics Metafile

A working demonstration of computergraphics metafile (cGm). the British andANSI standard that provides integration ofdifferent computer graphics products andthe exchange of information and imagesbetween systems. will be Oven at the

Computer Graphics Exhibition andConference from 7 to 9 November at theAlexandra Palace. London.

Simulator for Satellite Signals

A simulator that does a job similar tothose used to train aircraft pilots has beendeveloped by STC to check on the accuracyof Global Positioning Systems (GPs) usedworldwide for navigatine both civil andmilitary craft on land, sea and in the air.The company says that its STR2700 simu-lator will contribute to still more accuratenavigation from satellite signals.

Global Positioning Systems relay onsignals from 18-21 special navigationalsatellites in orbit around the earth, ofwhich the average user can 'see' up to fiveat any given moment.


EUlig CON-STF1

The valve eraOver the past forty years or so. communi-cation receiver design has undergone quitea revolution. In the days before transistorsand ics, there were some remarkably goodreceivers around. Typical among thesewere the AR88, the Hamerlund Super -Pro.the Marconi CR100, the BC348, and theRacal RA I7.

After the end of the Second World War.many radio amateurs were using eitherone of these classical designs or one of themany ex -military receivers that had comeon to the surplus market.

A large number of amateurs showedgreat ingenuity in the use of various itemsof military equipment to make up their sta-tion receiver, and sometimes their trans-mitter as well. There was a lot of ex -ser-vices expertise about and a considerableamount of technical discussion seemed totake place over the airwaves. Cobblingtogether all this readily obtainable gearwas not entirely caused by the non -avail-ability of proprietary amateur equipment:most of us being broke had something todo with it as well!

Towards the end of the valve era, thereoccurred a number of technical develop-ments in radio valve technology that had adirect bearing on communication receiverdesign. One of these was the appearanceof the frame grid pentode. like the E183.

These new valves. 465 kHz w trans-formers with a good Q. and ex -govern-ment quartz crystals, such as theFT241/243, helped to achieve respectableIF response shapes for reception of theincreasingly popular single-sideband (SSB )transmissions.

At the same time. wide -range, stableautomatic gain control (AGc) was becom-ing the norm, its control voltage no longerderived from the incoming carrier.

The emereence of the long -life stabledouble triodes, like the E88CC, originallydeveloped for the then embryonic comput-er industry, further helped to improveamateur communication receiver design.

Another milestone was the introductionof the beam deflection mixer valve, likethe 6AR8 and the 7360. which were devel-oped for the American colour Tv market.The remarkably linear mixing and large -

V CAI 0

by A. B. Bradshaw

signal handling capabilities of these newvalves soon caught the eye of receiverdesigners and it did not take long beforemanufacturers like Collins. Squires -Sanders. Drake. and so on were incorpo-rating them in their new receivers.

At about this time, a superb receiver,the Thomley G2DAF design. appeared onthe UK amateur scene. Many of theseexcellent receivers were built and had aprofound influence on our thinking ofwhat kind of performance could beachieved with the technology then avail-able. I built my own and well rememberthe pleasure of using the receiver. whichhad the knife-edge selectivity of theKokusai mechanical filter Type NIF455-10.K.

By then, we had the ingredients neces-sary for meeting the specification for agood communication receiver:

good IF shape factor (in spite of thelow IF resulting from the multi -conversionnecessary for the HF end):

stable conversion oscillators. necessaryfor the increasingly popular sss mode oftransmission:

ease of tuning with mechanical s/xtdrives (Eddystone 898, and so on).

Nevertheless, these receivers still hadsome serious short -comings. They werecomplex (at the time): they usuallyembodied lots of ganged switching oftuned circuits: they used relatively expen-sive wound components: their front-endalignment and tracking, particularly ingeneral coverage designs. was difficult:and lastly. these 'magificent machines'could certainly not be regarded asportable.

The solid-state eraThe transition to solid state electronicswas not a sudden occurrence, and forsome years hybrid designs were very pop-ular in the amateur press. Although thesedesigns still used valves in their front -ends, much of the remaining circuitry hadbecome solid-state. These early solid-statedevices, however, could not produce thegood intermodulation and cross modula-tion performance of their valved predeces-sors.

Over the past decade, solid-state

devices have improved enormously. how-ever, and present-day communicationreceivers have very real benefits comparedwith those of yesteryear.

Unfortunately, in my view, we haveallowed the Japanese industry to dominatethe manufacture and design of good -quali-ty communication receivers. This is partic-ularly disappointing in view of our ownearlier performance. In the solid-state erawe have manaeed to produce some inno-vative designs. but they are few and farbetween.

Nevertheless. the radio amateur re-mains in a unique position. The receivermanufacturer is hamstrung by severe eco-nomic restraints and market forces. Theamateur designer and constructor, on theother hand. is still at liberty to explore andindulge his fancy in ways that would heout of the question for the professionaldesigner. I am not suggesting for one mo-ment that the radio amateur can challengethe Japanese giants. Nevertheless. there isstill much innovation in Britain. well doc-umented in a variety of books, technicalarticles, application notes. and so on.

Modern home constructionIf we regard the modern communicationreceiver at a system level, we have a goodopportunity to see what some British man-ufacturers and suppliers have on offerRF amplifiers: Plessey Types SL600:SL611C: SL612: SL1610C: SLI611C:SL1612C.High performance mixers: Plessey TypeSL6440A/C (+30 dBm intercept point):Siliconix Type Si8901 double -balancedmixer (+35 dBm intercept point): variousdiode bridge ring devices, from theMD108 up to the SRA3 1£ 28 from Cirkit).IF shaping filters: ceramic and mechani-cal filters are available for the lower ws(455 kHz). while for the higher iFs thereare quartz crystal lattices up to 10.7 MHzare available in bandwidths suitable forAM, SSB and cw from Cirkit.IF amplifiers: three Plessey Type SL612tcs will give most of the gain required in anormal IF amplifier.Demodulators for AM, SSB, and cw:Plessey Types SL6700A and SL624.AGC generators: rather a limited choice


RADIO & TELEVISION

here, but the Plessey SL620 and SL621Ctcs are well proven.

This list shows that there is a goodhome-bred range of building blocks,although I still feel that there are areas ofdesign that have been neglected. Some ofthese are receiver front-end filtering forthe amateur bands, local oscillator design(either upper conversion synthesis or lim-ited -range BFO conversion systems), whilethe required noise floor specification forVHF synthesizers is a real challenge.

ConclusionAs I glance through yesteryear's copies ofRSGB Bulletin. RAD COM, and others, Ican not but be struck by the falling off ininterest in innovative design. Can thismalaise be halted? I certainly hope so.What are you going to do about it?

Encryption System May Be Basis forInternational Standard

A high security encryption system devel-oped by GEC Plessey Telecommunica-tions (GPT) for video conferencing. mayprovide the basis for a new internationalstandard.

Called B -Crypt. the system is sellingall over the world, demonstrating a stronginternational demand for a dedicatedencryption system.

B -Crypt operates on two 56 -bit securi-ty codes. The first is a crypto-variable keythat can be calculated automatically by thesy stem or input manually. The second isan initialization vector. This is a randomnumber that is changed and transmitted tothe receiver every 32 milliseconds. Theactual encryption key. a factor of thesetwo security codes. is never transmittedover the link and is. therefore. irretrievableby a third, unauthorized party. Whileencryption is active, all timeslots carryingvideo, audio and data are fully encrypted.Timeslots zero and two are unencrypted atall times for network signalling purposes.

Control of both the internal and stand-alone systems is by a terminal via a stan-dard RS232C serial port fitted to the rearpanel. Under normal circumstances, ini-tialization of encryption will occur whentwo suitably equipped locations of a videoteleconference request the transmission tobe encrypted via their local control termi-nal. If only one location makes the re-quest. encryption will not occur and awarning message will be displayed at thecontrol terminal. Both codec and stand-alone systems are fitted with 'Force En-cryption' capability. which enables them

References."Communication Receivers" by G.R.B.Thomley. RSGB Brrlletirt. July -Nov 1960and March -April 1961."An Amateur Bands CommunicationReceiver" by A.J. Shepherd. RSGBBu/le-tin July 1963."Prelude to a Communication Receiver"by I. Pogson. Radio & Hobbies(Australia) Sept. 1964 (2 parts)."The Deltahet Mark 2 Solid StateVersion" by I. Pogsen. Radio & Hobbies(Australia) Feb. 1971 (in 2 parts)."10-80 Metre Amateur Transceiver" byD.R. Bowman. Wireless World, June 1972."Defining and Measuring Receiver Dyna-mic Range" by W Hayward, QST. July1975."High Dynamic Range Receiver InputStages" by U.L. Rhode. Ham Radio. Oct.1976."Receiver Noise Figure, Sensitivity, and

ELECTRONICS SCENE

to react automatically to an encryptionrequest from a remote terminal with localintervention.

Another Step towards Global ISDN

British Telecom has taken another steptowards a global integrated services digitalnetwork (IsDN) with the introduction ofnew digital switched services linking theUK. Japan and the United States.

Customers of British Telecom. KDD andAT&T who have compatible equipmentwill now be able to use advanced applica-tions such as new generation (Group 4)facsimile, and the exchange of data tiles athigh speed.

The initial service will provide a circuitswitched data capability operating at56 kbit/s or 64 kbit/s. In time. this will beenhanced to provide supplementary ser-vices such as calling line identity andclosed user groups and functions such asterminal identification.

British Telecom's ISDN. named integrat-ed digital access-im-is available eitheras single -line or as multi -line.

Anglo-French-1 S Partners inGlobal Network

British Telecom. AT&T and FranceTelecom have signed a contract to providethe first phase of a communications net-work that will eventually span six conti-

Dynamic Range: What the Numbers\lean" by J. Fisk. Ham Radio, Oct. 1975."Optimum Design for HF CommunicationReceivers" by U.L. Rhode. Ham Radio,Oct. 1976."IF Amplifier Design" by U.L. Rhode.Ham Radio. March 1977."Noise Blanker Design" by \V. Stewart,Ham Radio. Nov. 1977."Effects of Noise on Receiver Systems"by U.L. Rhode. Ham Radio, Nov. 1977."The P\V Helford HF SSB Transceiver"by V. Goom. Practical Wireless. Nov.1980 (in 6 parts)."The RX80 Mk2" by A.L. Bailey. RadCom. Jan. 1981 (in 6 pans)."A Dual -Conversion Multimode ReceiverIF/AF Strip" by S. Niewiadomski, RadCont. May 1985 (in 2 pans i.".A Home Built Frequency Synthesizer for45-75 MHz" by J. Crawley. Rad Com.August 1986 (in 2 parts).

nents.By the end of this year. the first stage

of the project will link the three hubs ofLondon. New York and Paris, thus extend-ing the General Electric voice and datanetwork from the US into major Europeanlocations. British Telecom. from its net-work management centre in London, willbe the hub for the UK. the Netherlands,Ireland and Spain.

Other European locations will belinked next year. and phase 2 is planned tolink the Far East. Middle East and SouthAmerica. The global network will provideGE with voice, data and videoconferencingservices.

GPT Credit Card Payphonesfor Federal German'

Federal Germany's first public credit cardpayphone service, which uses Britishequipment, has recently gone into serviceat Frankfurt Airport. Others have beeninstalled at the airports of Dusseldorf.Hamburg and Munich.

The payphone equipment was devel-oped and supplied by GEC PlesseyTelecommunications (OPT) to the GermanBundespost. GPT has now supplied theequipment to 58 customers in 44 countriesworldwide.

The payphones accept payment bystandard international credit cards, such asAmex, Access. Diners and VISA.

Voice messages on the system are usedto assist the caller. if necessary. and areprovided with dual language capability.normally English plus the language of thecountry involved.

EI.EKTOR ELECTRONOCS SEPTEMBER 1989

24

STEREO VIEWEC.J. Ruissen & A.C. van Houwelingen

This electronic ornament is basically an unconventional VU -meter. Asquare matrix composed of 10x10 LEDs indicates signal volume as

well as stereo information.

The circuit is perhaps best qualified as asimple X -Y display for audio signals, andthe displayed patterns are, therefore, notunlike Lissajous figures. The heart of thecircuit is formed by an integrated circuitfrom National Semiconductor, the TypeLM3914. At first glance, this dot/bar dis-play driver is a quite conventional design.The IC houses ten comparators, a preci-sion linear -scale voltage divider and a ref-erence voltage source_ The actualrealization of these parts, however, givesthe LM3914 a number of interesting fea-tures:

outputs drive LEDs, LCDs, fluorescentdisplays or miniature bulbsexternal input selects bar or dot displaymodesimple to cascade for displays with aresolution of up to 100 stepsinternal voltage reference; adjustable be-tween 1.2 and 12 Vminimum supply voltage: 3 Vcurrent -regulated open -collector out-putsoutput current programmable from 2 to30 mAno multiplex switchinginput withstands Voutputs interface direct with TTL andCMOS logicfloating 10 -step divider can be connectedto a wide range of voltages, includinginternal reference

Circuit descriptionThe circuit diagram of the 10x10 LED ma-trix which determines the appearance ofthe stereo viewer is given in Fig. 2. Thedimensions of the matrix result in a squarearrangement. How the square is actuallypositioned is a matter of personal pref-erence, and not, of course, of any circuitconfiguration. The introductory photo-graph shows the prototype which has ma-trix co-ordinate X1 -Y1 below and X10 -Y10at the top.

The matrix arrangement allows anyone of the 100 LEDs to be turned on andoff individually. To select a particularLED, the relevant column, X1 -X10, androw, Y1 -Y10, is made high and low re-spectively. The circuit diagram of therow column driver (Fig. 1) shows thattwo LM3914s are used: IC2 forms the col-umn driver (X-axis), and IC3 the rowdriver (Y-axis). Both LM3914s are set to

operate in the dot mode so that, strictlyspeaking, one row and one column areselected to light one LED at a time. The ICoutputs have some overlap, however, sothat two LEDs are on at the switch -overlevels.

Transistors T2 -Tit function as inver-ters. They are required because the col-umn driver must switch to the positivesupply rather than to ground. The pro-grammable current source in 1C3 is set tosupply the relatively small base currentsfor the inverter transistors. The currentsource in IC2 is set to a much higher valueto supply the required current direct tothe LEDs.

The current source in the LN13914 is setin a rather unconventional manner: theoutput current is ten times the currentsupplied by the reference voltage. So, allthat is required is to load the reference

with a resistor. Rn sets the output currentof ICs to about 2 mA. A slightly differentapproach is used in the case of IC2: here,an LDR (light -dependent resistor), a tran-sistor, Ti, and a handful of other compo-nents form a load resistor whose value isa function of ambient light intensity. Sincethe output current of IC2 is used for driv-ing the LEDs, the display intensity is auto-matically controlled as a function ofambient light conditions. The componentvalues used allow the LED current to varybetween 8 and 25 mA.

To make sure the LEDs are completelyoff when they have to be off, the Ll out-puts of IC_2 and ICs are fitted with a pull-up resistor. This is required because theLl output has an auxiliary current sourcethat is used for cascading driver chips toform a larger display. The pull-up resis-tors keep Tr from conducting, and oneELEKTOR ELECTRONICS SEPTEMBER 1989

laSTEREO VIEWER

K2

K4

Pia

Al3

1 0 0 k %new

19 R3Y

I100n

6

R6

-0-

R5

5V5V

717p0V

6

22n

R8

TM10T

BC547A

PIED/

7

22n

R10

C5

C R9

5V

L10

L9iC2 Le

SIG 177

1:6LM3914 Ls

RHI L4

L3

REFOUT

REF CT

ADJ PLO

11

12

13

14

15

16

17

18

1

8 2

5V

4

100n

Ri

1M

LiL2

IC3 riSIG

L5

LM3914 LePHI 177

REFOUT lc§

REF L10ADJ R_O

R13

5V

R12

18

17

16

15

14

13

12

11

8 2 4

10T

10

CB

2p2 10V

©T3

K3

2 4 6 8 10 12 14 16 18 20O 0 0 O0bbb0O 0 0 0 0 0 0 0 0

1

4

(601'5

T6

5 7 9 11 13

9

T10

T11

15

T2...T11 = 10x BC559B

Al, A2 = IC1 = LM358

89C041 - 11

17 19

Fig. 1. Circuit diagram of the stereo viewer.ELEKTOR ELECTRONICS SEPTEMBER 1989

26 GENERAL INTEREST

Parts list

Fig. 2. Matrix configuration with 100 LEDs.

LED in the matrix from lighting, when theLT output is not actuated.

The audio signals applied to the stereoviewer are first attenuated to enable thedrive levels for IC2 and 1C3 to be set accur-ately. The sensitivity of the circuit is setwith potentiometer Pi. Acceptable drivelevels at the inputs are between 45 mVand 3 V.

01...D100

890044-12

The zero point of the matrix is shiftedto the centre of the square display with theaid of bias voltages on to which the AFsignals are superimposed. These voltagesare obtained with multiturn presets P2and P3, which are adjusted to supply halfthe reference voltage. A voltmeter is notrequired for this adjustment, because thezero indication can be seen to shift to the

Resistors (-.:5%):Fl1:R2;Ri3 = 10kR3;R4 = 1k5R5;R12 = 2k2R6 = LDRR7 = 1k8Rs = 1kR9 = 5600Rio =1k2Rii = 15kP1= 100k logarithmic potentiometer;stereoP2;P3 = 1 Mfg multiturn preset

Capacitors:Ci;C2 =220nC3;C5;Cio;C12 = 100nC4 = 47u; 10 VC6;C7 = 22nCa = 2112; 10 VCs = 1000A; 16 V; radialCii = 10g; 10 V

Semiconductors:131 = BY164

= LED; dia. 5 mm = BC547AT2-Ti i = BC559BICI = LM358IC2;1C3= LM3914IC4 = 7805

Miscellaneous: = 160 mA fuse with PCB -mount holder.Si = SPST mains switch.Tn = PCB -mount transformer 9 V; 1.5 A.Ki;K2 = phono socket.K.3 = pin header 2x10 contacts.K4 = 2 -way PCB terminal block.Ks = IDC header 2x10 contacts.PCB Type 890044 (see Readers Servicespage) -

Fig. 3. Track layout and component mounting plan.


centre of the display.The circuit is powered by a conven-

tional regulated 5 V supply, which isfitted on to the printed -circuit boardtogether with the associated mains trans-former.

Construction: simpleThe printed -circuit board shown in Fig. 3accommodates all parts except the LEDmatrix and the LDR for the display intens-ity control. Populating the PCB is entirelystraightforward if the wire links are in-stalled first.

The LED matrix is built separately ona square piece of veroboard. The installa-tion of the LEDs and the lozenge -shapedwiring at the rear of this board are greatlysimplified when the matrix is turned 45'with respect to the hole pattern in theboard. The LED matrix is connected via ashort length of flat -ribbon cable, for whicha mating 20 -way pin header, K3, is pro-vided on the main board.

The stereo viewer is simple to align:simply adjust P2 and P3 until the centrefour LEDs in the matrix are on. The sensi-tivity can then be set as required with theaid of the volume control, Pt.

Keep Con fi dental Meetings Private

A scanning radio receiver designed todetect even the most advanced eavesdrop-ping devices has been developed byAudiotel International.

This effective counter -surveillancedevice, named 'Scanlock 2000'. detectsnot only hidden radio microphones. butalso listening devices that use mainscables for transmission. It sweeps radiofrequencies from 10 MHz to 4 GHz anddetects Ast, FM and Sc (sub -carrier 20 kHzto 130 kHz) transmissions originatingwithin the room. It can be used to 'sweep'a room before a meeting and then left 'oneuard' during the meeting to detect any'bugs' that become active as a result oftimers or remote actuation.

In automatic mode. Scanlock locks tothe strongest local transmitter: even thelowest -powered 'bugs' in close proximitywill have a higher signal strength thanmore powerful but more distant legitimatetransmitters. Scanlock locks to a signal inunder half a second. In manual mode, theentire RF spectrum can be covered farmore rapidly than on a conventional radioreceiver owing to the advanced manner inwhich the unit monitors harmonicallyrelated frequencies simultaneously.

When a signal is located, the programbeing carried may be monitored directly

STEREO VIEWER

ELECTRONICS SCENE

on headphones or a built-in speaker: con-firmation that the transmission is from a'bug' nearby is obtained by the Scanlockemitting a 2 kHz tone and demonstratingthe presence of this tone in the transmis-sion being monitored.

Industry Prepares for the Videophone

INMOS expects to be the first manufacturerto offer a computer device that will allowvideophones to meet international stan-dards due to be agreed this year.

The company has recently launched itsnew high-performance signal processingdevice that is capable of rapid and high-

quality image data compression and de-compression for image handling applica-tions such as videophones and teleconfer-encing systems, photographic -qualitycolour facsimile, interactive video discsystems and high -definition colour imagesin desktop publishing.

Such systems require very high levelsof 'number crunching' to solve the prob-lems of digital data storage. communica-tion and reproduction. The new INMOS dis-crete cosine transform (oc-r) image proces-

sor is capable of 320 million operations asecond, providing computational powerfor the real-time orr calculations neededto compress video signals at data rates ofup to 20 MHz.

The INMOS IMS-Al2 I DCT image pro-cessor is housed in a 44 -pin PLCC package.Its initial price for quantities of 1000 ormore will be £47, but once volume pro-duction has started later this year. the priceis expected to fall to less than half this fie-ure.

Computer -displayed Artwork

A user's own artwork can be employed asthe basis of a computerized display andinformation system for educational. train-ing or promotional purposes by means of ahardware and software package fromCrystal Presentations.

The 'Electronic Display Unit' consistsof a touch -sensitive Al pad (594x841mm I to which the artwork is attached andthen connected to an IBM PC or compatible.

The system is used, for instance, byBritish Aerospace to familiarize pilots andmaintenance engineers with the warningannunciators on aircraft-flightdeck over-head panels.


CAPAC TO QS P'OR Rr APPLICATIONSA brief overview of currently available devices in an electronic

component group whose significance for telecommunications andradio circuits is often grossly underestimated.

Today's electronic component market of-fers a vast range of capacitors for use inhigh -frequency circuits. Many design en-gineers and home constructors are, there-fore, often faced with a real dilemmawhen it comes to choosing and mountingthe right capacitor in the right place. Inmany cases, parts lists of construction pro-jects will provide the value of the capaci-tor, but references like 'PTFE foiltrimmer', 'coffin -type leadless ceramic','ceramic NPO', 'tubular', and many more,may not be familiar.

Ceramic capacitorsThese small devices are probably the bestknown types for use in RF circuits becausethey are cheap and have been with us formany years. The lead spacing is usually2.5 or 5 mm for the modern disc and platetypes, of which some have rims below thecapacitor body to facilitate their fitting ata uniform height above the PCB surface(Stettner's 'Hot PantsT"' types). Valuesrange from 0.68 pF to 100 nF. Types withvalues greater than about 4.7 nF usuallyhave a working voltage of 63 V, or 12 V forsub -miniature types. Tubular capacitorsare rapidly becoming obsolete and are notrecommended for new designs.

The blue 'Sibatit' types from Siemensare often used as decoupling capacitors invideo and digital circuits because theyline up nicely with DIL ICs, require little

space and offer excellent RF and pulsecharacteristics. Sibatit capacitors must notbe confused with MKT types, which arealso blue but use a multi -layer polythera-phtelate dielectric structure. Popularvalues of Sibatit capacitors are 10 nF,47 nF and 100 nF. The terminal pitch isusually 5 mm, and the maximum workingvoltage 63 V.

Unfortunately, the value of the com-mon ceramic plate and capacitor is oftennot immediately evident from the print onthe device, so that a capacitance meter isrequired in case of doubt. Space restric-tions do not allow, say, 220p or 220pF tobe printed on the capacitor body. Instead,the value is printed as, for instance, 'n22',avoiding problems with a (tiny) decimalpoint, or confusion with trailing zeroes.Similarly, a value of, say, 47 nF, is oftenprinted as '473', meaning '47' with threezeroes: 47,000 pF.

The temperature coefficient of a ce-ramic capacitor is indicated by the col-oured band at the top of the body.Although many manufacturers deviatefrom the standard, a zero -coefficient, or'NPO', capacitor is generally marked witha black band. NPO capacitors are oftenused in oscillators to prevent temperaturechanges causing frequency drift.

FiltersPolystyrene and polypropylene capaci-

Feedthrough capacitors: screw -type heavy duty; ; solder type with eye: axial soldertype: ; low -capacitance feedthrough; 5 multiple feedthrough.

tors are fine for filters in audio and videoequipment and radio circuits (but not RFpower amplifiers) for up to 30 MHz. Alsoknown as 'styroflex' types (a trademark ofNorddeutschen Seekabelwerke AG, ofNordenham, Federal Germany) manufac-tured by Siemens, these capacitors areusually supplied with relatively thin,axial leads, although radial, plastic encap-sulated types are also available. The blackor red band on the white, greyish or silver -coloured capacitor body indicates the ter-minal that is connected to the outer foillayers. If applicable to the circuit, this ter-minal must be connected to ground to pro-vide a screening function. Specialpolystyrene types are available as parallelpairs for calibration purposes. These ca-pacitors have a non-standard, but accur-ately defined, value at a tolerance of 0.5%or better, which makes them eminentlysuitable for calibration of inductance andcapacitance meters.

The differences between polystyreneand polypropylene capacitors mainly en-tail the loss factor, permissible humidity,self -resonance frequency and insulationresistance. For most practical applica-tions, however, polystyrene and styroflextypes are all right. The version with thethin axial leads will be superseded byplastic encapsulated radial types becausethese have a fixed size and lead spacingand are, therefore, easier to handle inautomated PCB solder machines.

TrimmersCompression and mica foil trimmers aregenerally not used at frequencies above30 NtHz or so because their losses increaserapidly, and the Q (quality) factor dropsto an unacceptable level. Further, theirminimum capacitance is not low enoughfor tuned circuits in the VHF, UHF andSHF range.

Ceramic trimmers suitable for frequen-cies up to 500 MHz are available up to acapacitance of 100 pF. The maximum ca-pacitance is, however, nearly always lessimportant than the minimum capacitance,which is typically 15 to 305 of the maxi-mum value.

PTFE foil trimmers are often preferredto ceramic types because the insulatingfoil layers between the rotor and the statorare transparent. This allows the set capa-citance to be deduced readily from theposition of the grounded rotor bladeswith respect to the stator. The foil trim-mers from Valvo (Philips Components)are colour -coded to indicate the maxi -

ELL kTorz ELECTRONICS SEPTEMBER 1989

CAPACITORS FOR RF APPLICATIONS

*orart -

AIRfir

-

'07.1Qmiran

, I 'r -

1411.0111W

titiniaLiikv

Tubular trimmers for use in SHF circuits. i glass dielectrictypes: = PCB mount types (vertical); .72 PCB mount types(horizontal); chassis mounting types with screw and solderconnection; ; single hole mounting types.

mum capacitance: grey (5.5 pF); yellow(10 pF); green (20 pF); grey (40 pF); red(65 pF) and purple (100 pF). The dif-ference between the 5.5 pF and 40 pF typeis immediately apparent from the size.

Tubular ceramic or glass trimmers forchassis and PCB mounting are used inSHF circuits where line inductors oretched striplines are to be tuned at mini-mum loss. Well-known manufacturers ofhigh -quality air, glass and ceramic tubu-lar trimmers are Johansson, Tronser,Arco, Sky. Jackson and Stettner.

Decoupling capacitorsDisc- and coffin shaped leadless ceramiccapacitors are not so familiar among con-structors with little RF experience. Thesecapacitors are characterized by very lowinductance, which is mainly by virtue ofthe absence of connecting wires. Instead,metallized surfaces are used for solderingat both sides of the device. The coffin -shaped capacitor (called trapezoidal byStettner) is inserted and clamped in a4x0.7 mm PCB slot that keeps the devicein place during the soldering process. Theprinted circuit board tracks to the capaci-tor should be relatively wide to allow thelow inductance of the device to take effect.The value of the coffin -type capacitor isprinted on one metallized side. ValuesELEKTOR ELECTRONICS SEPTEMBER 1989

A selection of RF capacitors. 1 screw type feedthrough types;solder type feedthrough; 3 coffin -type leadless ceramic;

leadless chip; I heavy-duty ceramic holders for use in high -

power RF amplifiers; 6 feedthrough connection block.

range from about 5 pF to 2 nF at a maxi-mum working voltage of 63 V.

Disc -shaped leadless capacitors aremore difficult to handle because theymust, in general, be soldered on to a flatsurface. It is difficult, however, to heat oneside of the capacitor, and the surface itwill rest on, simultaneously. To ensure agood solder joint and sufficient adhesivestrength, a small hole is often drilled in thesurface, and some hot tin is appliedthrough it from the other side. In general,both the coffin and the disc -shaped lead -less ceramic capacitor should be handledwith utmost care because they are rela-tively brittle devices.

The smaller the better?The increasing use of surface -mount as-sembly (SMA) components has not gonepast unnoticed in the RF engineering fieldwhere the size of parts has always been acrucial factor. Not surprisingly, therefore,many RF circuits are currently producedin SMA technology. For capacitors in RFcircuits, this brings many benefits becauseSMA parts generally have a lower strayinductance than their normal -size equi-valents.

SMA capacitors must not, however, heconfused with chip types, which havebeen around for many years and offer

even lower stray inductance.

All photographs in this article have been reproducedcourtesy of Stettner & Co. GmbH Electronic Divi-sion PostfaCh 7 D-S55O Lauf West -Germany.

Trimmers. is tubular trimmers; ceramictrimmers. The tape at the centre containsSMA trimmers.

DD .P,C[1:31E METER

This portable instrument, designed by ELV GmbH, gives an accurateindication of the sound pressure level (SPL). The three SPL ranges,

three response modes, and linear or A -weighted filtering provided bythe meter enable many types of measurement to be carried out, fromthe tracing of ambient noise sources to establishing the sensitivity

of a loudspeaker.

Depending on their physical and mentalstate, human beings respond subjectivelyto ambient noise. Objective, absolutesound pressure level measurementstherefore invariably require a speciallydesigned test instrument, the decibelmeter, of which a design is discussed here.

Two weighting methods and associ-ated standard curves have been de-veloped and are widely used for soundpressure level (SPL) measurements. Oneof these methods yields the A -curve,which gives roughly the inverse of aver-age ear sensitivity. Figure la shows thestandard SPL response curves of the ear,and Fig. lb the frequency response of the

filter used for A -weighted measurements.A filter with A -type response is incorpor-ated in the present decibel meter to enableit to give readings which are meaningfulin terms of human response to soundpressure levels and variations thereof.

In a number of cases, it is useful toperform linear, that is, unweighted, SPLmeasurements. The SPL sensitivity of aloudspeaker, for instance, is established in

this manner across a frequency range of20 Hz to 20 KHz, with the decibel meterset to linear (unfiltered) response.

Controls and operationThe decibel meter has two rotary switchesand one slide switch on the front panel.

All functions of the instrument are se-lected with these three controls.

The slide switch selects linear (UN) orA -weighted (du t) response. In general, A -weighting is used whenever human re-sponse to sounds or noise is involved.This is so in practically all cases whereSPL measurements are used for determin-ing the degree of ambient noise. The un-weighted type of measurement, on theother hand, is commonly used for strictlytechnical work such as the design of aloudspeaker enclosure.

The right-hand rotary switch on thefront panel forms the range selector.When turned fully counter -clockwise toposition o, the analogue -to -digital (A -D)converter and the associated preamplifierin the instrument are tested. The displayshows a value between 00.0 and 00.4 (withthe left-hand rotary switch set to positionF). Higher values on the display indicate afault in the decibel meter. Position 70 dB

allows SPL measurements between 40 and70 dB to be taken; position 100 dB, meas-urements between 70 and 100 dB; and po-sition 130 dB, measurements between 100and 130 dB. Each range can be used forreadings of 10 dB below the minimumvalue or above the maximum value, withonly marginally lower accuracy. The

d8120

foo

800

50

D

0

20

oo

A-4444111pr"

i\\S -_-/-\-1V

thresholdf- I -1-i i

V 20 40 100 200 4C0Hz 1

102W/m210°

102

10

-6

108

1010

012

CI)

0

000

2 4 10 20kHzI -0-

899512-13

dB

-10

0

-10

-20

-30

-40

-50

-60

50 100 500 ik Ilk Ht

899512 - 15

Fig. 1. Equal loudness contours adopted for aural response testing (left) and the A -weighted filter characteristic derived from these (right).EI.EKTOR ELECTRONICS SEPTEMBER 1989

DECIBEL METER

\Pi UV

microphone preamplifier switch amplifier weightingfilter

log

rectifier &log. shaper

time `rieighting

ID 01-10_0 8_1.0

digitalreadout

899512-12

Fig. 2. Block diagram of the decibel meter.

measurement ranges thus have a suffi-ciently large overlap.

Accuracy of the decibel meter withinthe above three ranges (effectively from 40to 130 dB) is 0.5% typically. In most cases,the usable low and high limit is 35 dB and135 dB respectively, which corresponds toa dynamic range of no less than 100 dB, ora ratio 1:100,000.

The accuracy of the instrument is alsodetermined by the user who must select arange suitable for the relevant measure-ment before this is actually performed.The reading obtained during this testshould fall within the limits of the selectedrange_ If necessary, a higher or lowerrange is selected. Apart from small devia-tions from the minimum and maximumlevels of a particular range, readings thatfall outside the scale should not be used.This is because measurement errors riserapidly for readings more than 10 dBbelow the minimum or above the maxi-mum value of the selected range.

The left-hand rotary switch selects thespeed of the measurement, and also ser-ves to switch the instrument on and off.

In many cases, noise levels fluctuateand give rise to incorrect readings if thedecibel meter is not set to the appropriatespeed. The present instrument offers threespeed ranges for SPL measurements.

Slow (switch position s)A time constant of 1 s is used to ensure astable reading even when the SPL variesconsiderably. The slow meter response isparticularly useful for long-term meas-urements to give average sound levels.Fast changes in the SPL values are aver-aged out, while pulses are effectivelyeliminated.Fast (switch position F)In this mode, a time constant of 125 ms isused to enable fast SPL changes to be re-corded reliably. The display reading set-tles relatively quickly, and follows rapidSPL variations. This type of meter re-sponse is obligatory for certain peak-SPLmeasurements.Pulse (switch position p)Two time constants are used in this mode.Particularly suited to pulse and intensitymeasurements, this range uses a short risetime of 35 ms and a long decay time of10 s. It should be noted that meter over-flow may occur readily when the rangeELEKTOR ELECTRONICS SEPTEMBER 1989

setting of the instrument is changed. Ifthis happens, it is necessary to wait 10 suntil the measured value has dropped toa value lower than the expected SPL.In practice, the selection of a particularspeed response is determined by the ap-plication and the required read-out (peakvalue, average value, or fluctuations).

Block diagramThe block diagram of Fig. 2 shows the es-sential configuration of the decibel meter.The sound pressure to be measured is ap-plied to a high-grade electret microphone.This device, a Type KE4-211-2 from Senn-heiser, is constructed in back-electret tech-nology, and has an internal impedanceconverter. The element is housed in a TO -18 style transistor enclosure, and providesa good bandwidth (20 Hz to 20 kHz), ahigh signal-to-noise ratio, and a large dy-namic range (approx 35 dB to 135 dB). Thehack-electret technology ensures low sen-sitivity to vibration, and eliminates mech-anical resonance. All these features makethe KE4-211-2 an excellent choice for ap-plication in the present decibel meter,whose overall performance is certainly

C1011 C10i

I16Vu /2nKer

" c

R101

degraded if an electret microphone ele-ment with mediocre specifications is used.

The transducer and the associated pre-amplifier are housed in a tube that formsthe actual microphone. This constructionis necessary to ensure that the instrumentcan measure voltages of the order of 10µVin the most sensitive range.

An approximately 1 m long cable con-nects the microphone to the decibel meter.The use of separate units affords flexi-bility for the measurements, and also pre-vents the main instrument fromintroducing interference in the area to beexamined. The preamplifier is followedby the sensitivity selector which offersthree ranges as discussed above.

The next stage is a filter that allows aselection to be made between linear (flat)frequency response, and weighted re-sponse on the basis of the A -curve.

The amplified AF voltage is applied toa quadrature rectifier with a logarithmicshaper at the output. The rectified voltagemust be converted from linear to logarith-mic to enable the digital voltmeter (DVM)section to be driven in accordance with therequired scale curve. The time weightingsection between the output of the logarith-

1103 R102

IH 22K I470n

OP101, OP102=TL082=11101

1104

I---10pR103

22K 1-

6

-U58

OP131

4

0101

0X400®0102®l

DX400C105 I 1_4,

I 10pR105

47K 1-*

R104

OP102

70/0 *010 .%

AFmicrophoneconnections

ground

(seen from terminal side)

h

-U8

899512-14

Fig. 3. The microphone preamplifier is a two -stage design that uses a precision electretmicrophone from Sennheiser.

TEST AND MEASUREMENT

0 T

O

-IF

4 _

1:71-4-;1

-

aca,

0

L')

C")

00

Fig. 4. Circuit diagram of the main instrument. Note the use of temperature compensationthat acts on ADC IC3 and display driver IC4.

mic shaper and the input of the DVMallows selection of peak level measure-ment or a number of time constants forsignal integration.

Finally, the DVM and associated dis-play give a direct SPL reading in dB.

About the circuitThe circuit diagram of the microphone,i.e., the pressure transducer and associ-ated preamplifier, is given in Fig. 3. Themicrophone element is, in principle, ahyper -sensitive pressure transducer thatrecords air pressure changes caused bysound sources, and converts thesechanges into an alternating voltage. Thissignal is applied, via C103 and R102, to animpedance converter built around opampOPit-Ji. The gain of this stage is set to unitywith resistors R103 and Riot, while thephase shift is 180 degrees. The output sig-nal of OPioi, available at pin 7, is used forthe 130 dB range of the instrument. Therelevant connection is made via a PCBterminal marked `e'.

The two ranges with higher sensitivityrequire a further amplification of 10 times,which is achieved with OPit.)-2. and RIO-RI04. Two Schottky diodes with lowreverse current, Dim and Dit-c, are in-cluded in the feedback path of the secondopamp to prevent this affecting the oper-ation of the first stage when, for instance,the output of OP- reaches the clippinglevel as a result of high sound pressurelevels that may be measured in the 130 dBrange. In the lower ranges (70/100 dB) thesignal levels are so low that the diodeshave no effect.

Capacitors Ciar and C102 stabilize theamplifier and prevent it from oscillating.Resistor Rio] forms the load required forterminating the microphone element.

The power supply for the microphonecircuit is applied via terminals 'a' (posi-tive battery voltage) and 'c' (negative bat-tery voltage). Terminal 'b' connectsground of the microphone to the virtualground level created in the main instru-ment. As already discussed, terminals 'd'and 'e' form the signal outputs. The micro-phone is connected to the main instru-ment by a I In long 4 -way screened cable.The screening is connected to point 'b' onthe preamplifier PCB.

The circuit diagram of the main instru-ment is given in Fig. 4. Rotary switch Si.carries the AF signal from the microphoneunit to the input of the range amplifierbuilt around on. The required amplifica-tion factor is selected with 51b. The ratio ofthe selected resistor in the feedback net-work to Ri determines the amplification.Capacitor CI decouples the direct voltage.

Depending on the position of slideswitch 52, the AF signal supplied by OPiis fed either to the A -weighting filter, C2-C3-Cv-R7-RIO-R11, or, via Rs -R9, to invert-ing amplifier OP2, whose output signal iscapacitively coupled to the input of IC3.Capacitors C4, Cs, and C7 serve to de -couple the direct voltages.ELEKTOR ELECTRONICS SEPTEMBER 1989

DECIBEL METER

Circuit IC is an analogue -to -digitalconverter Type AD636. Originally de-signed as an rms-converter (Ref. 1), thischip has an on -chip linear -to -logarithmicconverter with a dB output. The dB con-verter is driven by the true-rms converter,and supplies an output voltage that corre-sponds to the logarithm of the rectifiedvalue. The logarithmic voltage availableat pin 8 of IC is applied to an invertingamplifier built around OP4. The amplifica-tion of this stage, which determines thescale factor, is set by RIn.

The position of time weighting selector53a determines whether the output voltageof OP4 is obtained direct (fast response),via 1-s time constant R ---C24, or via a peakrectifier built around OP:. In the lattercircuit, the charge time constant is deter-mined by RII-C15, and the discharge timeconstant by 1233-C15.

The selected voltage is fed to the inputof A -D converter ICs, an ICL7126. This ICconverts the potential difference thatexists between its pins 30 and 31 into acorresponding digital value, which isshown on an LC -display. The associatedreference voltage exists between pins 35and 36. An internal reference source sup-plies a voltage that is available at pin 32.This voltage is typically 2.8 V lower thanthe positive supply voltage (9 V in thiscase).

A special component in the displaydriver circuit is temperature sensor Tsi, aType SAX965. Resistors and R29 causethe regulation effect of the temperaturesensor to be such that the reference volt-age applied to pin 36 of IC4 varies in ac-cordance with the temperature of IC3, andtherefore with the voltage drift at the out-put of this ADC. This temperature com-pensation arrangement is required toensure the accuracy of the voltage at thedB output.

The compensation currents for thethree measurement ranges are changed tothe same extent as the reference voltage,but with a different sign. Inversion is ac-complished by 0113, which is connected tothe reference voltage via At an am-bient temperature of 25', Tsi drops a ref-erence voltage of about 470 mV.

The values of R21 and R25 are geared toachieve a level shift at the output of OR;that results in a difference of 30 dB be-tween the ranges.

The circuit is nulled with the aid ofpreset Ris, which feeds an additional (off-set) direct current into the lin-log conver-ter. The null adjustment is actually carriedout by selecting a measurement value to-wards the end of the range, and adjustingRis to obtain a reading that corresponds tothe applied input signal. The setting up ofthe decibel meter will be reverted to.

ConstructionThe complete circuitis built on two rela-tively small, single -sided, printed -circuitboards. Populating these in accordancewith the Parts List and the overlays inELEKTOR ELECTRONICS SEPTEMBER 1989

Fig. 7 is not likely to present difficulties.The low -profile components are

mounted first, followed by the highercomponents. Three parts are fitted hori-zontally: Cm on the preamplifier board,and Cu and Tsi on the main board.

Although it is, of course, best to mountthe temperature sensor such that it hasdirect thermal contact with IC3, acceptablecompensation is also achieved by the com-ponent placement on the PCB. This is sobecause the circuit is enclosed in a cabinetand will normally work at a fairly evenambient temperature. If the temperaturedrops suddenly by 10 degrees or more,about half an hour should be allowed forthe instrument to settle at the new am-bient temperature.

The following components are fitted atthe track side of the main PCB: R35, C1, Cu,C16-C20 and Cr, The terminals of Cu aresoldered direct to pins 4 (+ wire) and 11 (-wire) of IC2.

Fit soldering pins in the six holes forthe slide switch, S2, and cut the pins toabout 2 mm. The slide switch is securedby soldering its terminals to the pins.

The correct mounting height of the LCdisplay is ensured with the aid of a 40 -pinIC socket, which is first cut lengthwise.Next, the two 20 -pin rows are soldered onto the PCB. The drawing in Fig. 6 showshow a second IC socket is stacked on to theone on the PCB. This second socket is alsocut in two so that it can receive the pins ofthe LC display.

The solder terminals marked'h' on themain PCB are interconnected by a 35 cmlong flexible insulated wire installed atthe component side. Similarly, points 'j'are interconnected by a 25 cm long wire.

The wires of the battery clip are sol-dered to terminals 'f' (+; red) and 'g' (-;black).

The microphone wires, which are leftat their original length, are inserted fromthe component side in the holes providedin the preamplifier board. The wires arethen soldered at the track side, and after-wards bent at right angles where theyleave the printed -circuit board at the com-ponent side. This ensures that the micro-phone points forward so that its face canprotrude from the tube to be fitted later.

The preamplifier board is ready formounting inside the approximately

Step Range (dB) 1 kHz test signal(Urns)

1 100-130 631 mV = 63.1 Pa

2 100-130 20 mV s 2 Pa

3 100-130 631 mV s 63.1 Pa4

5

6

100-130 20 mV s 2 Pa

70-100 20 mV E.- 2 Pa

40-70 631 AV = 0.063 Pa

Table 1. The setting up procedure

100 mm long metal tube after the 4 wiresand the screening of the cable to the maininstrument have been connected to theappropriate solder terminals 'a' through'e'. The small PCB is then inserted into thetube to a point where the top of the micro-phone element protrudes about 3 mmfrom the front side of the tube. A heavy-duty soldering iron is then used to soldera part of the ground surface of the pream-plifier PCB to the inside of the metal tube.This solder joint should be made quicklyand as far as possible to the rear of the tubeto prevent overheating the sensitive ele-ment at the front side.

The remainder of the construction en-tails connecting the 4 wires and the cablescreening to the main instrument, and in-stalling this in the enclosure providedwith the kit.

Setting upAn ammeter is inserted between the com-pleted instrument and the 9 V battery. Theleft-hand rotary switch is set to positionOFF, the right-hand switch to position 0,and the slide switch to position LIX. Thecurrent must be nought, and the displayblank.

The left-hand rotary switch is turned toposition F (fast). The resulting current isbetween 4 and 10 mA (6 mA typ.). Thedisplay should indicate a value between00.0 and 00.4. If not, the circuit should beexamined for faulty solder joints and in-correctly placed components, particularlyin the area around IC2 and ICs.

The right-hand rotary switch is ad-vanced to the 70 dB position, when theoperation of the instrument can be veri-fied by saying a few words into the micro-phone.

Now connect a sine -wave generator ca-pable of supplying a 1 kHz output signalat low distortion to the temporarily usedadjustment circuit shown in Fig. 5. Thewiper of the 1 140 preset is connected tojunction Rini-Cio3 on the preamplifierboard by a short length of screened wire(the screening is connected to the groundplane on the preamplifier PCB). Themicrophone terminal to the above junc-tion is temporarily removed. The sine -wave generator must have a separatepower supply.

Rx (Fig. 5) I Indication Adjust with

wire 130 dB

39k 100 dB

wire 130 dB

39k 100 dB

39k 100 dB I

R18

R16

R16

1M 70 dB _J

34 TEST AND \IEASCREMENT

Parts list

MAIN INSTRUMENTResistors:Rs = 47052R2,4 = 1k

R25 = 2k551:15:R31 = 3k3

R9 = 6k8Ri;R3 = 10kRio = 12kR7 = 15k

= 22kR17 = 27kRs = 47kRi4;Ris = 68kR2o;R23:R24;1:125;R27:R3o;R32:R34:R38 = 100kR1s:R21;R36 = 180kR35 = 220kR2;R25 = 330kR22 = 680kR12:R13;R33;R37 = 1MS2Rio = 50k multiturn preset

= 100k multiturn preset

Capacitors:C23 = 47pC14 = 1n0C6 = 1n5C22 = 22n ceramicC2.;C1o;C13;C16;C17:C1s;C2i= 47nC2;C1a = 100nC2 = 220nC7;Ca;C1i,C12 =1140; 16 VC1;C4:C5;C15;C23;C24 = 10µ: 16 VC25 = 100p: 16 V

Semiconductors:IC = TL081IC2 = TL084

= AD636IC4 = ICL7126 = 1N4148

Miscellaneous:TS1 = SAX965.312 -digit LC display.SI:S.3.= 4 -way, 3 -pole rotary switch for PCBmounting.S2 = miniature DPDT slide switch.Clip for PP3 battery.15 solder pins.1 m 4 -way screened cable.2 40 -way DIL sockets (for LCD mounting).2 5 -mm PCB spacers.2 self -tapping screws.

MICROPHONE PREAMPLIFIERResistors:R1c1;RIc4 = 4k7R1e2;R1o3 = 22kR1C5 = 47k

Capacitors:C1c4;C1c5 = 10pC102 = 22n ceramicC103 = 470nClot = 101.i; 16 VSemiconductors:ICtoi = TL082D1o1;Dlo2 = DX400Miscellaneous:5 solder pins.Microphone Type KE 4-211-2 (Sennheiser).Metal tube.

Fig. 5. Component overlays and top views of the completed printed -circuit boards.ELEKTOR ELECTRONICS SEPTEMBER 1989

DECIBEL METER

decibel meter.

The three dB ranges of the instrumentare calibrated by simulating the voltagessupplied by the microphone element. Theadjustment procedure is given in Table 1.The left-hand rotary switch remains in po-sition F, and the slide switch in positionLIN.

The first step in the alignment proce-dure is the fitting of a shorting wire inposition R, and setting preset RA to anoutput voltage of 631 mVrm.s. Preset RI., onthe main board is adjusted to achieve adisplay reading of 130 dB. A real zero -calibration is not possible with Ris be-cause the meter ranges do not reach downto 0 in actual use. The alternative is to shiftthe zero -point upwards and thus achievethe reading that corresponds to a particu-lar sound level, which may be simulatedby a test voltage of the correct amplitude.

The second step is the adjustment ofthe scale factor, with the aid of Rio, toachieve a reading of 100 dB when the pre-amplifier is driven by 20 niVrm..; (fit a39 kit resistor in position IL). This adjust-ment may shift the previously set null -point a little, so that the third step in theadjustment entails repeating step 1 (apply631 mV77 and adjust Ris for an indicationof 130 dB).

For the fourth step in the adjustment,drive the preamplifier with 20 niVrim andadjust Rib for a reading of 100 dB. Theabove steps are repeated until the 100 dBand 130 dB readings are obtained whenthe corresponding voltage is applied.

The range setting is turned to the 70-100 dB range, which is tested with the aidof a 20 mVrms signal. The circuit is dimen-sioned such that the display automaticallyindicates 100 dB at a maximum toleranceof 0.5 dB. \o adjustments are required forthis range.

The same goes for the 40-70 dB range,which is tested by applying a voltage of631 01. The display should read 70 dB±0.5 dB.

Deviations exceeding the above toler-ances require the resistor values to bechecked carefully. If desired, the differen-ces between the ranges can be eliminatedby making small changes to R23 (100 dBrange) and R2I or Ra2 in the 70 dB range.These resistors are temporarily replacedwith multiturn presets, which are ad-justed for 100 dB and 70 dB when theELEKTOR ELECTRONICS SEPTEMBER 1989

1 KHzsine - wavegenerator775 mVeff

RA

1K to R101/C103

RB

470

ground lb)

899512-16

5 mm PCBspacer

psiCl at track

side

s2

PCB

31/2 -digit LCD

IC socket

IC socket

899512-17

Fig. 6. Auxiliary circuit for setting up the Fig. 7. Side view of the main board. Note that the LCD is mounted on to two stacked ICsockets, which have been cut lengthwise to give the correct width.

1 kHz test signal at the correspondinglevel is applied. The presets are thenremoved, their resistance is measured,and each is replaced by a resistor, or acombination of resistors, of the requiredvalue. In general, however, these correc-tions will not be necessary.

It will be clear that the previously de-tailed setting -up procedure yields validresults only when the microphone ele-ment has a sensitivity of 10 mV /Pa. Inpractice, however, a tolerance of ±2.5 dBshould be taken into account. To preventthis degrading the accuracy of the decibelmeter from the stated 0.5 dB, the sensitiv-ity specifications of each microphone ele-ment have been tested by ELV. The devicefound in the kit is supplied with a notestating the correction factor, which cantake a value between 0.7 and 1.4. The testlevels given in Table 1 are to be multipliedwith this correction factor.

A brief example is given to illustratethe level computations. Assuming that themicrophone element has a correction fac-tor of 1.2, the test voltage level for the firststep in the alignment procedure must be

631 mV x 1.2 = 757.2 mV.Similarly, the second step is to be per-formed with a test voltage of

20 mV x 1.2 = 24 mV, etc.Finally, it should be noted that the testvoltages should be measured with a volt-meter that is known to give accurate read-ings at a frequency of 1 kHz.

Final assemblyThe sine -wave generator and the level ad-justment circuit of Fig. 5 may be discon-nected if all tests and adjustments arecompleted satisfactorily. The screenedcable is disconnected from junction Rioi-C103, and the connection to the micro-phone terminal is restored. Say a fewwords aloud to check that the instrumentworks in all ranges, including the dBAfunction and the three response modes.

Fit the PCB of the main instrument intothe upper half of the hand-held enclosure,and secure it on to the battery compart-ment panel with the aid of two 2x10 mmself -tapping screws. Ensure the correctdistance between the PCB and the insideof the enclosure by two 5 mm long spa-cers. At the other side, secure the PCB

with a few drops of two -component,epoxy resin or super -glue.

Pass the cable to the microphone unitthrough a 4 mm hole in the top wall of theenclosure. After mounting the strain -re-lief clamp, connect the 4 wires and thescreening to the appropriate points on thePCB. Care should be taken not to cause ashort-circuit via the screening connection.The enclosure may now be closed with therear panel.

The preamplifier PCB in the metal tubemust be protected from ambient influen-ces. This may be achieved by filling thetube with a suitable resin or potting com-pound. Great care should be taken not tospill resin over the microphone element,whose face should be temporarily coveredwith adhesive tape. Then carefully insertremovable potting between the micro-phone and the inside of the tube. The com-pound should go a few millimeters intothe tube.

Hold the tube vertically, and pour theresin in at the cable side. After about24 hours, remove the compound aroundthe microphone element, and carefullypour resin into the microphone side of thetube until it is full. The face of the micro-phone must be 2 to 3 mm above the topside of the tube. The protective tape maybe removed after the resin has hardened.The decibel meter is ready for use.

Reference:

1. "True -RIM Meter". Elektor ElectronicsDecember 1986.

A complete kit of parts for the DecibelMeter, which is designed in West -Ger-many, is available from the designers'exclusive worldwide distributors (re-grettably not in the USA and Canada):

ELV FranceB.P. 40F-57480 Sierck-les-BainsFRANCETelephone: +33 82827213Fax: +33 82838180

Also see ELV France's advertisementelsewhere in this issue.

PC AS TONE GENERATORJ. Schafer DL7PE

A GW-BASIC program and a few modifications to the loudspeakercircuit enable any PC, whether an XT, AT or compatible, to function

as a precision tone generator with a frequency range of 20 Hz to120 kHz, with a basic sweep function as a useful option. The nicething about this generator is that it costs next to nothing, while

doubling as a frequency meter.

The present PC tone generator, which isreally a BASIC program only, is ideal foraligning a wide range of AF circuits. Thefrequency of the generated tone can be setaccurately, so that the low-cost tone gen-erator is suitable for applications that in-clude the tuning of musical instruments(electronic tuning fork), the aligning ofRTTY, fax and SSTV filters, and thedimensioning and testing of many othertypes of tone decoder. In many cases, theBASIC program obviates the use of a func-tion generator and a frequency meter.This is of particular interest for applica-tions in the audio range, where frequencymeters are, in general, not very accurate.The PC tone generator allows AF frequen-cies to be defined with an accuracy of afraction of a hertz.

PC as tone generatorApart from the possibilities offered byBASIC commands BEEP and SOUND, thereexists a more powerful way of generatingtones with the aid of a personal computer:direct control of the relevant hardware.This enables frequencies to be generatedat quartz -crystal stability in the rangefrom 20 Hz to several hundred kHz, inde-pendently of the PC's clock frequency.The lower frequency limit is fixed, but theupper limit can be made as high asallowed by the PC's internal AF amplifier.The relevant BASIC commands may befound in lines 260 through 330 of the list-ing.

The frequency resolution is excellent,especially in the audible range: at a basicfrequency of 10 kHz, the step size is assmall as 85 Hz, or 0.85(1-; between 2 and3 kHz, the step size is 5 Hz (0.16" ); andbelow 1 kHz it is 0.1 Hz (0.01% ).

The PC generates the required tone viathe built-in loudspeaker. This is adequatefor nearly all calibration and adjustmentwork in the acoustic range. An oscillator,for instance, is simple to calibrate accur-ately by means of a beat -frequency meas-urement in which the PC functions as thereference. Just compare the two tones bylistening to them simultaneously, andstep the PC tone frequency until the dif-ference frequency decreases. When itbecomes inaudible, the frequency gener-

ated by the equipment under test can beread from the PC screen. Similarly, the PCtone generator can be set to a particularfrequency, so that the oscillator can beadjusted until zero -beat is achieved.

If the generated tone is required elec-trically also, the loudspeaker signal mustbe made available on a jack socket - seeFig. 1. When a plug is inserted, the loud-speaker in the PC is automatically dis-

Fig. 1. Coupling out the PC's AF signal via a jack socket with a break contact.

100n

06...9V

LM386

50k

6 mem 47p5 10V

0

A.F.

10 0p10V

Lad100n

S.890064 I 12

Fig. 2. The 'active' alternative: raising the PC's AF signal with the aid of a small amplifier.


PC AS TONE GENERATOR

abled, and the generated tone is availableat a relatively low impedance. The 8 SIresistor protects the internal AF amplifieragainst short-circuits. The value of thisresistor must be increased as required ifthe internal loudspeaker is a high -imped-ance type. It is, of course, also possible towire the socket such that the loudspeakeris not disabled, but taken up in series withthe output signal. This arrangement obvi-ates the use of the protection resistor. Byconnecting a 16 SI resistor instead of theindicated 100 S1 type to ground, the gener-ated tone is simultaneously audible viathe internal loudspeaker.

A further possibility is shown in Fig. 2.A small amplifier with adjustable volumeis connected to the jack socket. This solu-tion is particularly useful for PCs thathave an internal loudspeaker with a rela-tively high impedance, or one that pro-duces insufficient output volume.

The programThe frequency range of 20 Hz to 120 kHzis fixed in lines 180 and 200 of the theBASIC program. Keys are used to controlthe program:

Key 'e': enable toneKey 'd': disable toneKey '+': increase frequency by previouslyentered step sizeKey '-': decrease frequency by previouslyentered step sizeKey 's': terminate program

A frequency sweep is obtained by holdingthe + or - key - the tone frequency thenincreases or decreases at the previouslyentered step size.

Applications

Here are a fev of the many possible appli-cations of the computer -controlled tonegenerator:

test signal for aligning RTTY circuits,e.g., 1275/2125 Hz for VHF stations, and1275/1445 Hz for SW stations.test signal for tone decoders1,000 Hz frequency referencetuning forkelementary acoustics

Table 1 is useful for the tuning fork appli-cation because it shows the tone frequen-cies for three octaves.

E PR:Nt FF:NT "

GOS1:5 --:l5 COLL'R :F126 LOCA7E130 LOCATE, ...... =7

:- PE!".- Gona Generator

:NR= " please er.ter a=s__ frequency :";FREOplease Enter step size :-.STP

P= t FREOUENCY = "; USING'

Xi=:NR-7 Xi -"N FREO SIP:F GOSUB 260

:F .7EN :ED:-G

- EE0GOTO 250

Zr... "L:ad 1E2 fr:m timer bloc..: "

FE:! :!sta f:r :i7er block-" tyte"-R" "7:-==--='.gr.5=imant byte"

EEM "supply leE,Et-significant byte""s7-TLY -:z_-significant byte"

70NE 0ENERATO R:2: -by 0L -FE -

RETURNGeG LOCATE 22.1200FPINT"

":PRINT:FRINT "key functions -COLOR 0.7

-1- PRINT" UP ;COLOR 7,0

-E FR/NT " - ":::LOR 0,7FR -"7 GC14N

EE

El

EE

7.0" d

ZGLOR 7.0...-NT - s ;

_OR 0.7FRINT" STOP ";::.OR 7,0LEGATE 5,26PINT -range: 20 Hz to . 100 kHz"RETURN

97.1NP(9.7) OR 2 :REM turn on PC loudspeakerRETURN

97.INP(97) AND 252 :REM turn off PC loudspeakerRETURN

Note

C

0#

4th octave 5th octave 6th octave

261.6

277.2

293.7

523.3 1046.5

554.4 1108.7

587.3 1174.7D# 311.1 622.3 1244.5E

F

329.6 659.3 1318.5

349.2 698.5 1396.9F# 370.0 740.0 1480.0G

G#

A

392.0

415.3

440.0

784.0 1568.0

830.6 1661.2

Table 1. Commonly used frequencies for A#

880.0 1760.0466.2 932.3 1864.7tuning musical instruments.


H 493.9 987.8 1975.5

\I 0

by Roy C. Whitehead, C.Eng., AILEE

This article describes some simple transmission -line experimentsthat were developed for the UNCLE scheme. Under this scheme,which was initiated by the lEE, but later joined by other learned

societies, members (usually retired) volunteer to go to schools tohelp teachers to bridge the gaps that exist between the academic

world and the world of practical engineering.

The material used in the experi-ments consisted of:

a known length of coaxialcable of which both ends wereaccessible:

a twin -beam oscilloscope:an HF generator with 75 SI

output:three 100 fI non -inductive

potentiometers:an ohmmeter.

Measurement ofvelocity ratioThe equipment should be con-nected as shown in Fie. 1. Set Ptto its maximum resistance valueand the two Y sensitivity controts of the CR0 to produce equalvalues of Y sensitivity.

Set the signal generator to itsminimum available frequencyand note the small lateral dis-placement of the two wave-forms. Then, increase the gener-ator frequency, which causes thelateral displacement of thewaveforms to increase. until, forthe first time. the two wave-forms are seen to be in phase.The propagation time of thecable now equals one period t =1/f of the generator output. Thevelocity ratio of the cable thenequals

Fig. 1.

velocity in cable / velocity in free space =

= cable length in metres x f/ (3 x 108).

A typical value is 0.8.

Change the generator frequency to onequarter of the value used previously,which makes the line one quarter wave-length long. Adjust P1 to obtain vertical Ydeflections on the CR0 of equal magnitude.

Fig. 2.

Disconnect Pt and measure its effectivevalue. which is equal to the characteristicimpedance, Zo. of the cable.

Measurement of attenuation/frequency characteristicsConnect the equipment as shown in Fig. 2.Set Rt to equal Z0. Potentiometer PI hasbeen provided with a decibel scale (whichcan be done with the aid of the ohmmeter).

Adjust R2 to provide across theinput end of the line an impe-dance equal to Z0. If the outputimpedance of the generator is 75CI, this will be 43 O.Over a range of frequencies, sayfrom 100 kHz to the maximum atwhich the available equipmentwill operate satisfactorily. adjustPt to produce Y deflections ofequal magnitude. The cable atten-uations are then equal to theattenuations of the potentiometer.The attenuation/frequency char-acteristic of the cable roughly fol-lows the emperical equation

loss = (asf+ bf ) [dB]

where b«a so that the secondterm becomes significant only atfrequencies above about 16 MHz,owing to the skin effect.If a loss/frequency equalizer bedesigned and constructed. thismay be tested in a similar man-ner. after which the line plus theequalizer may be tested.Other tests may also be carriedout. For instance, short-circuit theoutput end of the cable at Yt andnote the effects on the Y2 wave-form for odd and even numbersof quarter wavelengths. Repeatthis test with an open circuit atYt.The relationships between cable

length and the frequencies at which theCR0 and generator can operate satisfactori-ly should be noted. The shorter the cable.the higher must be the operating frequen-cies of the generator and the CRO.

The connectors used should preferablybe coaxial. otherwise they should be short,especially when operation is at frequenciesabove 10 MHz.


ANALOGU

Most applications for analogue switchesfall into two categories: signal routeingand signal conditioning. Different process-ing technologies produce switches withdifferent characteristics. One advantage ofthe new CMOS analogue switches fromSiliconix is that they allow you to controlsignals that fall anywhere between the twopower supply rails. Furthermore. these

. '

0 31n

Fig. 1. The DG400 series of analogue switches.

high-performance silicon -gate ics. theDG400 family. arc pin -for -pin replace-ments for the popular DG200 series. Theyoffer significantly lower on -resistance(rDs(on )=85 II). lower power dissipation135 1.1W 1. faster switching speed (ton=250 ns) and lower leakage current astofft<500 pA) than the older industry -standardparts. The new devices are shown here intypical circuits, illustrating the benefitsthey offer.

Sample -and -hold functionsIn most data acquisition systems, man)channels are sampled sequentially andthen digitized by an analogue -to -digitalconverter. In choosing or designing a sam-ple -and -hold system. speed and accuracyare the two most important considerations.

Open -loop, cascaded -follower sample -and -holdcircuitThe basic sample -and -hold circuit ofFig. 2 has unity -gain buffers to charge thecapacitor without loading the signal sourceand to drive the next stage without chang-ing the voltage stored. The basic operationof this circuit is illustrated in the photo -

The authors are vith Siliconix Ltd.

rRATSW

OrTCHES

by Jack Armijos and Tania Chur

10 k

12

3 IInput

flr

OP -27

-12 V

Sample:Hold Control

V -12 V

-12 V

.12 V

octp.it5

-12 V

= 2200 oF Poistrene

Fig. 2. Open -loop sampleandhold circuit uses a fast analogue switch.

Fig. 3. A logic Low opens the DG412, placing thecircuit in the hold mode.

graph of Fig. 3. This configuration pro-vides fast acquisition times and is good forhigh-speed acquisition systems.

The input buffer is chosen for low off-set voltage, good slew rates, and the abili-ty to drive the capacitive load. A polysty-rene capacitor is used because of its verylow dielectric absorption and low leakage.The output buffer needs to have a shortsettling time and very low input bias toprevent the discharge of the hold capacitorduring the hold mode.

The most important switch parametersare: speed. to minimize the acquisitiontime (fast throughput): low charge injec-

Fig. 4. Vout showing the effects of charge injec-tion.

lion. to reduce the hold step error: and lowleakage, to maintain a low droop rate. TheDG412 offers improvements for all threeareas of performance.

The circuit shown in Fig. 2 achieved anacquisition time of under 900 ns and adroop rate of 10 µV/µs. Pedestal error wasa function of analogue signal voltage. Theworst -case error was 23 mV when Vin =5V.

The photograph in Fig. 4 shows Vontimmediately after the hold command. Inthis case. Vin = 0.5 V. Note the upsetcaused by the charge injection of theswitch when it opens. the offset error that


40 COMPONENTS

.5V

Sarni)ILIUMContra!

.1511

-15 V

Fig. 5. Integrator output sample -and -hold function operates switch into virtual earth.

remains after the upset and the droop ratethat begins after settling is completed.

Closed -loop integrator out-put sample -and -hold circuitA popular sample -and -hold configurationis shown in Fig. 5. This circuit is >impleand accurate. It has a gain of -1 since R1= Opamp A I acts as a current boosterto speed up the charging rate of holdcapacitor CH. Since the unity -gain buffer

has a very low output impedance. the timeconstant associated with CH is determinedprimarily by the on -resistance of theswitch and by the magnitude of the holdcapacitor. Thus, the circuit benefits greatl\from the low on -resistance of the DG411.

The settling time of the output voltageis determined by the slew rate and settlingtime of the integrator stage. In the samplemode of operation, the DG41 1 closes andhold capacitor CH charges to the negativeof the input voltage. In the hold mode, the

V..

Sample/Hold Control

.5V .15V

12 111

113 114

= -15V

Fig. 7. Fast and precise sample -and -hold circuit.

Fig. 8. Vow without compensation shows largeglitches and a waveform ripple during acquisi-tion time.

i4

.15V

15000PvlYstlionv

IM

IVedlv (A) SM

VV-3.

51/

IV

-5V

(B) Vow

1.71

vI.v2- 2.324VII kw, -1.911V

So I'll- 1.711INIft

;114

Fig. 9. Improved Vint after compensation.

Fig. 6. Acquisition time is limited by the slewrate of the output amplifier.

analogue sv. itch opens after the capacitorhas acquired this voltage to the desiredaccuracy. Another advantage of theDG411 is that the switch always operatesat a virtual earth potential regardless of theinput voltage. Since at this level thecharge injection on the switch drain is atits minimum value, the hold step error isminimized. The errors of Al are mini-mized in the sample state. although theydo appear in the hold mode.

The photograph in Fig. 6 shows thetypical waveforms associated with this cir-cuit. With the components shown, this cir-cuit achieved an acquisition time of about20 !As. a maximum hold step error of3.8 mV and a droop rate of 7.5 ItV/ps.

Fast and precise sample -

and -hold circuitThe circuit shown in Fig. 7 u1.2 a DG404analogue switch in conjunction with a JFETinput operational amplifier. The DG404 isa fast switch (Ton<150 ns). In this circuit.both switches have a similar potentialwhen open. so their charge injkectioneffect is minimized by their differentialeffect on the opamp. Acquisition time ofthis circuit was less than 600 ns. worst -

case pedestal error was -5 mV. and drooprate was 35 11V/ps.

The compensation network formed byCC and RC helps to reduce the hold -timeglitch and optimizes acquisition time. Thephotograph in Fig. 8 shows this circuit'soutput without a compensation network.Notice the large glitch going into the holdmode. as well as the rippled waveformright after the output slews to its newvalue at settling time. The photograph inFig. 9 shows the improved response afterthe compensating network has been instal-led.


A NEW GENERATION OF ANALOGUE SWITCHES

.15 V(A) Data In

e15 V

16 11

1076411

4 -15 REF

17

I I

3

T33 gF

1 I 2.

OF -27

-15 V

.50 e15 V

11110

4

Trick/Held

01411

.15V

1640

-1-11100

3 7

1030I

-15 V

-152z--1511110

Fig. 10. Digital -to -analogue converter deglitcher.

S

Digital -to -analogueconverter deglitcherMajor code transitions in digital -to -ana-logue converters (DACs) can cause unwant-ed voltage spikes, commonly called glitch-es. In many DAC applications. these glitch-es can not be tolerated. Additionally. DACSfrom different vendors have different sizeglitches. (Note the glitch impulse specifi-cation on DAC data sheets). To ensure asmooth transition when the DAC goes fromone voltage to the next and to guaranteeuniform circuit response regardless ofalternate -sourced DACS. the DAC outputmay be processed with a tack -and -hold asshown in Fig. 10. While the DAC inputcode is unchanged. the DG418 is closedand Vout tracks the output of the current -to -voltage converter. Just before a codechange occurs, the analogue switch isopened so that Vout continues showing theprevious voltage. After the code changeand its associated glitch has settled, the

DG418 closes again and the track mode isresumed.

The photograph in Fig. 11 shows Voutwith the DG418 always closed (c) andwith the deglitcher active (d). Notice theimprovement in the transition glitches.

The DG418 offers high switchingspeeds. which are required for short con-version times. and low charge injection,which minimizes pedestal errors.

Dual -input programmablegain amplifierFor digital systems where only a +5 Vsupply is available, a small amount of ana-logue processing can be implemented witha low -voltage converter ic and low -volt-age analogue components. Figure 12shows an amplifier suitable for data acqui-sition or voice recognition applicationswhere either of two analogue signals isselected and amplified by a very precise.self -calibrating chopper -stabilized amplifi-

101;.F

109c11

Input 2

Io1I2e

7

16

.5V

11 112

.1FF 0.1 pF

15

.5V

1/206423

317510

-5

5 VT0 F

14

317652

-5V

5

A.2X 2

Select10 P'

1/2 man

vi,

5E1

4211

1t0

5eV -

(B) Traclutiold

rf-16. a.. a..-.

71gwV (C) Vou, with the switch closed

leen"Sty

'.4

V (0) Vow deglitched

Fig. 11. Digital -to -analogue converter deglitcherwaveforms.

er. Circuit eain can he -.elected as either x2or x10. A single DG423 analogue switchwas used to perform both the input -selectand gain -select functions. Its low on -resis-tance. high speed. and on -chip latches easecircuit design and improve overall accura-cy.

The photograph in Fig. 13 illustratesthe operation of the circuit. For demon-stration purposes. input- and gain -selectswere tied toaether so that when the 0.5 V(p -p) triangular signal was being pro-cessed. the circuit vain was x10 whereaswhen the 3 V (p -p) sine wave was select-ed. the amplifier's gain was reduced to x2.This type of gain ranging is useful to pre-condition analogue signals of differentamplitudes prior to an analogue -to -digitalconversion.

Fig. 12. Lowvoltage programmable gain amplifier.


Fig. 13. Gain ranging produces similar ampli-tudes even if the input levels are different.

42 COMPONENTS

+5V

13---4,,,,F1' +5 V

.01 2

Me"+1 -r---

-

15

V1,-5 ci,

Clod = 22C0 pf i

11 Select. ---->--26

1/2 74221

Fig. 14. Programmable one-shot multivibrator.

Fig. 15.. A logic LOW produces short pulses anda logic I- , 31i creates long ones.

06119

Fig. 16. This photograph illustrates theswitch -over action.

remote

tarn

VI ,4 5V, C

= C I ... F1 o3o s,

6

IM

0611f

3 713 -

3

S

10 kill

10 30 R

Vett

Fig. 17. Remo:e SP DT analogue switch for switched signal powers.

Rad GA

Fig. 18. DG411s in the head switching circuit of a disk drive.

Programmable one-shotmultivibratorAnother useful application for an analogueswitch, a programmable one-shot multivi-brator, is shown in Fig. 14. This circuitproduces pulses whose duration is deter-mined by digitally selecting one of the twotiming resistors-see Fig.15. Advantagesof the use of the DG419 in this circuit are:small size (8 -pin minion:, or small -outlinepackage), high speed, low on -resistance.and TTL. compatibility even in single -sup-ply operation.

Analogue switch poweredby input signalThe analogue switch in Fig. 17 derivesoperating power from its input signal. pro-vided that the amplitude of that signalexceeds 4 V and the frequency is greaterthan 1 kHz. This circuit is useful whensignals are to be routed to either of tworemote loads. Only three conductors arcrequired: one for the signal to be switched,one for the control signal and a commonreturn.

A positive input pulse - see Fig. 16 -turns on clamping diode DI and chargesC 1. The charge stored on the capacitor isused to power the chip: operation is satis-factory because the switch requires a sup-ply current of not greater than 1 p.A.Loading of the signal source is impercepti-ble. The DG419's on -resistance has therespectable value of 100 f for an inputsignal of 5 V.

Read/write disk -drive circuitThe circuit shown in Fig. 18 allows data tobe written to or read from a disk. In thewrite mode. SW2 is closed. A ONE is cre-ated by momentarily closing SW1. Thiscauses current to flow in the left-hand halfof the head coil. A ZERO is produced whenSW3 is closed. This causes current to flowin the right-hand half of the coil and re-verses the direction of the maenetic flux.

In the read mode. switches SW4 andSW5 are closed. This connects the headcoil to the read preamplifier so that thevoltages picked up by the head as the diskglides by can be amplified.

Single -supply operation with +12 V.low -on resistance and high switchingspeed allow an improvement in data ratesof roughly x10 when DG4I Is are used inplace of the more mature DG211s.

Micropower UPS transferswitchThe purpose of the uninterrupted powersupply (uPs) circuit in Fig. 19 is to pre-serve volatile memory contents in the


A NEW GENERATION OF ANALOGUE SWITCHES

Ya

(.5

wan

313 to

Maori

V.

SW

3

3Milan -

CtU f

Fig. 19. Micropower uPs circuit.

event of a power failure. In this applica-tion. every tenth of a volt counts. This cir-cuit uses a micropower analogue switchthat comes in an 8 -pin minim or small-

outline package. a 3-V lithium cell to sup-ply back-up power. a diode and two resis-tors. Voltage losses under 0.1 V can beachie-ved.

During normal operation. currents ofseveral hundreds milliamperes are sup-plied from Vcc. In this mode, SW1 isopen. so that the only drain from the lithi-um cell consists of leakage currents flow-ing into the V1 and S terminals. The leak-age current is typically about 10 pA.Resistors R1 and R, are continuouslysampling Vcc.

When Vcc drops to 3.3 V, the DG417changes states, closing SW1 and connect-ing the back-up cell. Diode D i preventscurrent from leaking back towards the restof the circuit. Current consumption by thecstos analogue switch is around 100 pA:

this ensures that most of the availablepower is applied to the memory where itis really needed. In the stand-by mode.currents of some hundreds of milliam-peres are sufficient to retain data.When the +5 V supply comes back on, thepotential divider senses the presence of atleast 3.5 V and causes a new change ofstate in the analogue switch, restoringnormal operation.

On -resistance is about 74 S2 when Vccis +5 V and 128 S2 when Vcc is +3 V. Forexample. an 800 l.tA load. equivalent to astatic RAM of 256 kbit (M0v161L16), willproduce a voltage drop of 0.1 V on theanalogue switch, which is much betterthan the 0.6 V drop occurring if a simple2 -diode circuit were used.Higher currents and lower losses can beachieved by paralleling several sections ina multiple analogue switch such as theDG403.

Line a in the photograph in Fig. 20

Fig. 20. Oscilloscope waveforms show a cleanpower switch over.

illustrates how, in spite of Vcc droppingto V (line b). uninterrupted power is ap-plied to the load. Negligible voltage lossis caused by the switch. Line c shows thatthe DG417 changes state when its controlinput voltage decays to 1.4 V andchanges again when it reaches 1.5 V onits way back to normal. The values of R1and R2 may be adjusted for different trippoints if desired.

For the applications mentioned in thisarticle. the DG4090 family of silicon -gateCMOs switches comes a step closer to theideal switch. Any application that usesindustry -standard analogue switches cannow be improved by choosing these fast,lower -power. versatile analogue switches.

Course for the Radio Amateurs'Exam - May 1990

There will be a course for the May 1990Radio Amateurs' Exam at Newark Techni-cal College. Chauntry Park. Newark,Notts. starting this month on Mondayevenings from 7 to 9 p.m. The course tutoris Alistair Morrison G4YZG.

Further details may be obtained fromBert Drury GIUMK at the college, tele-phone (0636) 705921.

Radio Amateurs' Course

North Trafford College, Manchester isagain offering a Radio Amateurs' Coursestarting this month. The course will com-prise:

Theory

Morse Code

Thursday evening orWednesday morning.Tuesday evening or

ELECTRONICS SCENE

Wednesday afternoonAmateur TV Wednesday morningAdvanced MorseCode Monday evening

Lecturer: J.T. Beaumont G3NGD.

Enrolment dates are 6, 7 and 8 September.Further details from North TraffordCollege of Further Education TalbotRoad Stretford Manchester M32 OXH Telephone 061 872 3731.

New Showroom for Nevada

Nevada, the specialist suppliers of Com-munications. Music and Discothequeequipment have opened a new showroomin Portsmouth. adjacent to their present

premises at 189 London Road. North End,Portsmouth P022 9AE. telephone (0705)662145.

In the new showroom, radio, scannerand short-wave enthusiasts will be able tobrowse in comfort with full 'hands on'facilities over Nevada's expanded range ofproducts.

New Maplin Store

Maplin Electronics. one of the fastestgrowing electronic retail groups inEurope, has recently opened another storein London.

The new store, managed by LawrenceSaunders. is located at 146-148 Burnt OakBroadway. Edgware. Middlesex.

Further details from Maplin HeadOffice. telephone (0702) 552911.


1iJ

TA MODEL1/1by T. Wig -more

The sixth part in the series deals in detail with the Booster Unit.Each of these amplifiers provides enough power for the control

of up to fifteen trains on a digital model track. The booster is the lastunit in the series that can be used with both the Marklin and the

Elektor ElectronicsDigital Train System. The units featured inforthcoming parts in the series are peculiar to the

Elektor Electronics System.

The power supply of a digitally controlledmodel railway track is fundamentally dif-ferent from that of a conventional track, inthat the supply voltage is switched rapidlybetween +18 V and -18 V. The switching iscarried out by the booster (power amplifi-er).

The booster ensures that the serial con-trol commands generated by the digitalcontrol circuits contain not only the infor-mation, but also the power to start locomo-tives, turnouts (points) and signals.

Since derailments, and the consequentshort-circuits of the track, occur muchmore often on model railway tracks thanon life-size ones, it is essential that thebooster is provided with an efficient short-circuit protection facility.

The conceptOur booster unit has two important advan-tages over that from Marklin: higher out-put power and a regulated output voltage.

The Nlarklin booster provides a maxi-mum output current of about 3 A. That isnot much if vou take the current drawn byone locomotive at about 700 mA , and addto this the current drawn by turnouts(points), signals, and coach lighting. It ison those considerations that our boosterprovides an output current of 10 A.

The output voltage of the Marklinbooster is fairly load -dependent: a 25%drop over the normal range of loads isquite normal. That kind of variation has, ofcourse, an adverse effect on the speed ofthe locomotives and the brightness of thecoach lights.

The output stage is an emitter follower.Driving the bases by a voltage sourceensures a virtually constant output volt-age, which results in independent speedcontrol of the trains and constant bright-ness of the various lights. These propertiesare illustrated in Fig. 39 and Fig. 41.

The use of an emitter follower alsoenables higher switching speeds since thetransistors operate on the linear part oftheir characteristics: the switching timesare, therefore, not adversely affected bysaturation effects.

A drawback of the configuration is thehigher voltage and the consequent greater

dissipation in the output transistors.Fortunately, this is easily rectified by theuse of somewhat larger heat sinks.

The circuitSince the switching pattern of the trackvoltage contains control information, it isimportant that the booster provides a cleanoutput signal. Much attention has, there-fore, been paid to the switching speed. Thepractical outcome is illustrated in Fig. 40.

The bases of the emitter follower, T1-T4in Fig. 42, are switched by Ts and Tsrespectively between +20 V and -20 V.These voltages are provided by ICI-D3and IC2-D4 respectively. The final outputvoltage is the difference between the basevoltage and the sum of the base -emitterpotential (about 1.5 V) of the output tran-sistors and the drop across the emitterresistors (maximum 0.6 V). In practice, the

output voltage is a reasonably constant±18 V. See also the load characteristic inFig. 41.

The emitter follower ensures a betterbandwidth and regulation with complexloads than, for instance, feedback.

Emitter resistors R12-R15 ensure anequal division of current to TI-T2 on theone hand and to T3-T4 on the other

Resistors R12 and R14 serve to measurethe current in aid of short-circuit protec-tion transistors T9 and Tio. When the emit-ter current of Ti or T3 tends to become toohigh, the drop across R12 or R14 rises suffi-ciently to switch on T9 or T1D. This causes areduction in the base current of the outputtransistors and, consequently, in their col-lector and emitter currents.

The input stage is formed by T. and Tsand is configured in a manner that makes asymmetrical input signal essential. If theinput (pin 4 of Kt) is 0 V or not connected,

flo

4

Fig. 38. The booster unit without heat sinks and enclosure.

ELEKTOR ELECTRONICS SEPTENIBER 1989

GENERAL INTEREST

Fig. 39. Data transfer by switching the supplyvoltage. It is evident that the Marklin Booster(upper trace) does not provide a regulated out-put in contrast to the Elektor Electronics unit(lower trace).

all transistors are switched off and the out-put presents a high impedance (that is, novoltage is supplied to the rails). When theinput voltage is between +5 V and +20 V,T7, 1.5, Ti, and T2 conduct and the output isswitched to +18 V. With the input voltagebetween -5 V and -20 V, T3 and T4conduct and the output voltage is switchedto -18 V.

All transistors, except T5 and To, oper-ate on the linear part of their characteris-tics. Transistors T5 and T6 are switched inthe saturation region because switchingtransistors for voltages of 50 V and moreare not available. Nevertheless, Cs ensures

that these transistors switch at a sufficient-ly high speed.

Overload signalThe circuit around Tn serves to indicate anoverload condition. Note that only thenegative output voltage is monitored. Thisis sufficient since the load on the negativeline is slightly higher than that on the posi-tive rail. For instance, the turnout (points)decoders work with half -wave rectifiersand, therefore, load only the negative rail.Moreover, when no data are being trans-mitted, the output voltage is negative.

When the booster is overloaded, T9 andTio limit the current in the first instance.The output voltage will then drop signifi-cantly and this causes a rise in the voltageacross the output transistors and thus inthe dissipation. If this situation is allowedto persist, there is a danger of the boosterbeing thermally overloaded: the conse-quent risk of fire is a very real one.

Therefore, if the output voltage dropsbelow 15 V, Tii will switch off. The signalat pin 5 of Ki, aided by the pull-up resistoron the main PCB of the Elektor Electronicssystem, goes low and this results in theremoval of the drive to the booster withthe aid of the software. Thermal overloadsare, therefore, prevented; moreover, thesystem 'knows' that in this condition nodata can be transmitted (even if they

imam.5"ZIE111111111

1111111111111

Fig. 40. Comparison of the switching behaviourunder load of the Marklin Booster (upper traces)and the Elektor Electronics unit (lower traces).

UC! f

2

INNIIIMIIIIRIIIMII I,

lila 1

1 ..,

MI -.

1

_

H

Fig. 41. Load characteristic of the booster unit.

could, they would not reachthe decoders).

Capacitor C enables theoverload action to be delayed,so that the system is not dis-abled at every momentaryshort circuit. This will bereverted to later in the series.

20V 20V

2x 01N4148

0 00

VI

09

804658131/64 30 m

*see tea:

1N4148

az

8C5578

low 2 5 V

T1,1-2 = 00V65

137 iN4001X

03 T9

iN400iir08 "

3009 404

1N4001

IAC6

D10 , 01 = 1N4148T3 , T4 = BDV64

Fig. 42, Circuit diagram of the booster unit.

023-30V

Ra

23-30V

S722 I %I 41

Constructionli the Po) shown in Fig. 43 isused, construction of thebooster unit should not pre-sent any problems.

Fit the wire links first:those close to the output tran-sistors should be of 1 mm dia.wire.

Mount resistors R12-Ri5well away from the board,because they get pretty hotduring operation.

The board has provisionfor a 5 -pin DIN connector, but ifthe booster is intended for usein a stationary position (whichis normally the case), therespective wires may be sol-dered direct to the board.

Circuits ICI and IC2 donot need a heat sink.

Do not fit C7 at thisstage.

Transistors T1 -T4 mustbe mounted on a heat sinkwith a thermal resistance ofnot less than 0.8 KAY with theaid of good -quality insulating


46 THE DIGITAL NIODEL TRAIN - PART 6

Fig. 43. Circuit diagram of a recommended power supply.

washers. If BDX66/67 darlingtons (withTO -3 housing) are used, they must first bemounted on to the heat sink and then con-nected to the board by not too long, heavyduty wires.

Power supplyThe circuit diagram of a recommendedpower supply is shown in Fig. 43. The2x18 V transformer must preferably be atoroidal type. The rectifier must be aheavy-duty type and needs a heat sink (itmay be mounted on to that for T1-T4).

If you do not want to go to the expenseof a new transformer, but rather use onethat vou have had lying around for sometime, use the circuit shown in Fig. 44.Remember, however, that such a set-upwill normally not be able to deliver morethan a quarter of the power of the supplyin Fig. 43, if that.

Finally, DO NOT connect transformers inparallel to increase the total available cur-rent: such a set-up can be a death trap.

Fig. 44. If you are content (for the timebeing) with a much smaller outputcurrent. this power supply will donicely.

Assembly and testSince the booster is operated from themains, great care and attention must bepaid to correct assembly and insulation.Because there are ahvays metal parts in amodel railway system that can be touched(like the rails), it is advisable to use a good -quality insulated enclosure.

The insulation of the power supplytransformer stated in the parts list isapproved to Class 1. This means that themains cable should have three cores, oneof which is earth.

All metal parts that can be touched(including the heat sinks) should be con-nected to earth.

Connect the two secondary windings ofthe mains transformer in Fig. 43 in seriesand fit and solder the rectifier and thebuffer capacitors (Ctoi = Ci + C2 = C3 + C4= 20,000 uF rated at > 40 V).

Before the supply is connected to thebooster, switch on the mains and checkthat the direct voltage across the buffer

01"

capacitors is ±25-29 V. If you measure 0 V.it is almost certain that the two secondarywindings have been connected in anti -phase. Switch off the mains, discharge thecapacitors via a resistor and reverse theconnections of one of the secondary wind-ings. Then check the direct voltage again.

If everything is all right, switch off themains again and discharge the buffercapacitors via a resistor.

Next, connect the supply to the boostervia insulated wire of at least 03 mm dia.and switch on the mains. Check that the

Parts list

Resistors:131;R2 = 18 kR3;114= 2k2Rs;Rs = 4k7R7:Fi8 = 100C1; 1 WRe = 10kRio;Rti = 1k0Ri2(196Ris = 0015; 4 W

Capacitors:C1;C2 = 10p; 25 VCa:C4 = 220nCs:Cs =10Oµ; 40 VC7 = 68p; 16 VCa= 10n

Semiconductors:Di:D2;Dio:Dvi = 1N4148D3;Da:D9 = zener diode 15 V; 400 mWD5-Ds = 1N4001T1;T2 = BDV 65 (Philips Components)T3;T4 = BDV 64 (Philips Components)T5 = BC640T6 = BC639T7 = BC547BTa = BC557BTs = BC337Tic) = 8C327Tn = BC557ICI = 7805IC2 = 7905

Miscellaneous:Kt = 5 -way DIN socket (1800) for PCBmounting.5 off car -type spade terminals for PCBmounting.Insulation material forPCB Type 87291-6 (see Readers Servicespage).

Recommended power supply parts (not onPCB):Mains transformer: 2x18 V @300 VA (e.g.ILP 73014)Smoothing capacitors: 4 off 10,000pF; 40 Vor 4 off 15,000µF; 40 V.High -current bridge rectifier: min. 20 A (e.g.,BYW61 from Motorola).One mains -rated fuse: 2 A slow.Two low -voltage fuses: 10 A fast .Heat -sink: e.g., SK120-100mm (Dau Compo-nents; Fischer).

Fig. 45. Printed circuit board for the booster unit.

El.EKTOR ELECTRONICS SEPTEMBER 1989

GENERAL INTEREST 47output voltage of ICI is +20 V and that ofIC2 is -20 V. There should be no voltagebetween B (earth) and R since there is asyet no input.

Fit a 100 0, 5 W, resistor between B andR and connect the input, pin 4 of Kt, to apositive voltage, for instance, +20 V at pin3 of Ki. The output voltage should then be+18 V. With a negative input - obtained byinterconnecting pins 1 and 4 of Kt - theoutput should be -18 V.

Connecting to Marklin DigitalThe booster circuit is driven via Kt, whichalso carries the auxiliary +20 V and -20 Vvoltages. These are not of importancewhen the Marklin Digital is used, but inthe Elektor Electronics system they powerthe RS232 interface.

When the Marklin Digital is used, onlypins 2 (earth) and 4 (input) of Kt need tobe connected to the brown and red termi-nal at the rear of the Central Unit. Ourbooster, therefore, does NOT use the 5 -pinconnector on the Central Unit.

The overload signal (pin 5 of Kt) is alsofor use with our own system only. Toarrange for the automatic switch -off of theMarklin unit during overloads, a diodemust be added as shown in Fig. 46.Warning of a short circuit in the booster isthen passed to the Central Unit, afterwhich the current monitor in that unitarranges the switch -off.

The Central Unit may provide some ofthe power to the rails, but note that onlythe B connexions of the Central Unit andour booster may be interlinked. The R con-

IEE Meetings3-8 Sep - Personal and mobile radio sys-

tems (Vacation School at Swansea).4-7 Sep - Microwave (Conference at

Wembley Conference Centre).5-8 Sep - Circuit theory and design

(Conference at Univ. of Sussex).17-22 Sep - The application of artificial

intelligence (Vacation School at Univ.of Reading).

17-22 Sep - Data communications andnetworks (Vacation School at AstonUniversity).

17-22 Sep - Integrated electronic prod-uct design (Vacation School at Univer-sity of Surrey).

18-20 Sep - Software engineering forreal-time systems (Conference atCirencester).

18-22 Sep - Quantum electronics (Con-ference at University of Oxford).

24-30 Sep - Microwave measurement(Vacation School at Univ. of Kent).

Further information on these. and manyother, events from IEE Savoy Place LONDON WC2R OBL Telephone01-240 1871.ELEKTOR ELECTRONICS SEPTEMBER 1989

Fig. 46. How to connect the booster unit to Marklin's Central Unit.

flexions (centre rail in the Marklin system)must be isolated from one another.Marklin supplies special parts to preventthe slide contacts from short-circuiting theelectrically separated centre rails duringcross-overs.

For true -to -scale modellersThe output voltage of the booster was cho-

The Third International Symposium onIC Design and Manufacture will be heldon 14-15 September in Singapore. Detailsfrom the School of Electrical and Electro-nic Engineering Nanyang TechnologicalInstitue Nanyang Avenue SINGA-PORE Telephone Singapore 2263.

A series of short courses on Graphics andImage Processing. Microprocessors.Signal Processing. Supercomputers.Computer Networks. Voice & DataCommunications, and many others, willbe conducted by ICS Trafalgar House Hammersmith International Centre LONDON W6 8DN Phone 01-748 6667.

A seminar on Mid -range systems and'UNIX will be held at the QEII Centre,Westminster. London on 27-28 Septem-ber. Details from Blenheim Online

sen at ±18 V to ensure that the maximumspeed of the locomotives would be aboutequal to that in traditional model railwaysystems. Taken to scale, model trains trav-el faster than life-size ones. Modellers whowant to have their locomotives travel at,proportionally, the same speed as life-sizetrains can arrange this by using 12 V zenerdiodes in the D3, D4 and D4 positions. Thiswill result in an output voltage of =15 V

Blenheim House Ashill Drive PINNERHAS' 2AE Telephone 01-868 -1466.

A number of seminars has been organizedfor this month by Frost & Sullivan. Sub-jects include: Information Technology:Telecommunications & Data Communi-cations: and Electronic Engineering.Details from Frost & Sullivan SullivanHouse 4 Grosvenor Gardens LONDONSW I W ODH Telephone 01-730 3438.

Synopses of papers for the Internation alConference on Electromagnetic Compa-tibility (University of York: 28-31 August1990) should be submitted before 29 Sep-tember: those for the Fourth Internatio-nal Conference on Advanced InfraredDetectors and Systems (London:5-7 June1990) before 29 August; and those for theInternational Conference on IntegratedBroadband Services and Networks(London: 15-18 October 1990) before 15January to: Conference Services 1EE Savoy Place LONDON WC2R OBL Telephone 01-240 1871 Ext. 222.

48

INTERMEDIATE PROJECTA series of projects for the not -so -experienced constructor. Although each article

will describe in detail the operation, use, construction and, where relevant, theunderlying theory of the project, constructors will, none the less, require an

elementary knowledge of electronic engineering. Each project in the series will bebased on inexpensive and commonly available parts.

5. Resonance meter

J. Bareford

The present intermediate project follows this month's theme byventuring out into the fascinating world of radio techniques. The testinstrument discussed is a must for anyone working with RF signals,

but with a limited budget. It enables the resonance frequency oftuned circuits to be measured within the range of 100 kHz to about

50 MHz, and can also be used as a capacitance meter, RF testgenerator and RF signal probe.

Traditionally, the name of the instrumentof the type to be described has evolvedfrom grid dipper to gate dipper or simplydipper. The first name, grid dipper, wasused in the valve era and long after_ Whenthermionic valves disappeared from con-sumer electronic equipment, the instru-ment was built from semiconductors and

baptized 'gate dipper' because the gate ofa field effect transistor (FET) is electricallyvery similar to the first grid of a valve. Theinstrument is basically an RF signalsource with adjustable output frequency,coupled to a circuit that measures andindicates the amplitude of the output sig-nal - see Fig_ 1_ Because the terms 'gate

and 'grid' have been formed historically,but have really nothing to do with thebasic function of the instrument, thesemisnomers are omitted here to be re-placed by the more universal term 'reson-ance meter'.

Tuned circuits andresonanceMany constructors shy away from pro-jects that contain home-made inductors,because these, they feel, remain some-thing of a mystery owing to their lack ofexperience or suitable test and measuringequipment. And yet, many a radio ama-teur will confidently inform these con-structors that there is nothing mysteriousabout winding coils. In fact, dimensioningthem and peaking the resultant tuned cir-cuit at the right frequency is sheer plea-sure, provided, he will tell vou, that aresonance meter is available. Without thissimple instrument even experienced RFengineers are often at a loss in gettingradio equipment to work correctly.

Any tuned circuit absorbs energy fromanother that is placed near it, and reson-ates at the same frequency. The RF energyis supplied by the resonance meter and aninductor that forms part of an oscillator.When this inductor is held near the coilunder test, the oscillator output ampli-tude drops if the two tuned circuits reson-ate at the same frequency. When the 'dip'is indicated by the signal level meter onthe resonance meter, the resonance fre-quency of the tuned circuit under test canbe read from the tuning dial. Mind you:


RESONANCE METER 49

Fig. 1. Block diagram of the resonancemeter.

the resonance frequency can be deter-mined while the equipment of which thetuned circuit forms part is not powered.The coupling between the resonancemeter and the tuned circuit under test isentirely inductive: all that is required is tohold the pick-up coil on the meter close tothe tuned circuit under test. Tune the res-onance meter, and the signal level meteron it will tell you the resonance frequencyof the L -C network under test.

Resonance meter as an RFsignal source...Since the resonance meter contains an os-cillator capable of covering a fairly largefrequency range, it may double as an RFsignal generator. To align a receiver, forinstance, the resonance meter is simply setto the required frequency and placed closeto the aerial input. If it is too strong for aprecise adjustment, the test signal can beattenuated by placing the resonance meterfurther away.

... as a frequency meter orRF probe...The resonance meter is designed such thatit can easily be used as a coarse frequencymeter and signal strength meter (RFprobe). These functions are achieved byswitching off the internal oscillator, butleaving the pick-up coil and the signalrectifier plus level indicator in function.Energy picked up from a resonating in -

f (MHz) L (H)

0.1-0.2 10m

0.2-0.45 2m2

0.45-1.0 470µ

1.0-2.0 100g

2.0-4.5 22p.

4.5-10 4g7

10-20 1µ

15-40 01122

Table 1. The values of La.

Fig. 2. Circuit diagram of the budget resonance meter for 0.1-50 MHz in eight ranges.

ductor in equipment to be aligned thuscauses the meter to deflect when the tun-ing dial is set to the correct frequency. Themeter indication is a measure of the signalstrength, while the tuning dial shows themeasured frequency. These combinedfunctions are particularly useful for alig-ning receivers and transmitters in whichseveral frequencies are mixed. The probefunction of the resonance meter is thenideal for, say, aligning the filter that fol-lows the mixer stage, so that only thewanted frequency is passed.

...and a C or L meterCapacitance (C) and inductance (L) meas-urements are the last additional functionsof the resonance meter.

The value of a capacitor can be deter-mined with the aid of a parallel inductorwith known inductance, L, and, of course,a resonance meter. The capacitance, C, issimple to calculate from the resonance fre-quency, Jo, of the parallel tuned circuit:

1

21tdLC

Since the self-inductance is known, andthe resonance frequency can be measured,the equation can be rewritten as

1C=40 f5 L

Similarly, inductance can be calculatedwith the aid of a reference capacitor:

1

L 40fcC

Three transistorsThe circuit diagram of the resonancemeter is given in Fig. 2. All functions dis-cussed above are realized by three transis-tors and a handful of passive components.Although perhaps a little difficult to de-duce from the circuit diagram, Ti and T3form an oscillator. The frequency of oscil-lation is determined by La and varicaps Diand D2. The two diodes are connected inparallel to achieve the required capacit-ance range that can be adjusted with Pi.

A total of eight plug-in inductors isrequired to cover the frequency rangefrom 0.1 to 50 MHz.

Preset P2 allows the collector current inboth transistors to be adjusted, givingcontrol over the amount of RF energygenerated by the oscillator. Transistors Tiand T2 form a differential amplifier in whichC7 provides the feedback between the col-lector of T2 and the base of Ti.

The measuring amplifier is formed byT3. This transistor is operated in class C,so that it does not conduct until the volt-age on L3 is about 0.6 V higher than theemitter voltage. This means that T3 formsa basic rectifier because it conducts onlyduring a part of the positive half -wave ofthe oscillator signal. This pulsating signalis converted into a dean direct voltage byCl. Regulator ICI prevents fluctuations ofthe supply voltage degrading the stabilityof the oscillator. Should the supply volt-age be unstable, the voltage at Pt, and withit the varicap voltage, is unstable also. Thevaricap voltage, by the way, is not onlydependent on the setting of Pt. Presets P3and P4 are included to give Pi the correctrange, which is a must for the calibrationof the scale on the front -panel designed


50 INTERMEDIATE PROJECT

Fig. 3. Printed -circuit board for the reson-ance meter.

Parts list

Resistors (-±5%):Ri;R7= 1k0R2 = 22052R3 = 47k1:14;R5= 33kRs = 220 kPi = 100k linear potentiometerP2 = 50k linear potentiometerP3;P4 = 5k preset H

Capacitors:Ci = 100nC2 = 22nC3 = 47µ; 6 V; radialC4 = 220nC5 = 1µ0; 16 VC6 = 1µ0; 25 VC7 = 39n

Semiconductors:Di;D2= BB212Ta;T2 = BF451T3 = BFR91ICI = 78L10

Miscellaneous:PCB Type 886071 (see Readers Servicespage).

= 1mHO radial choke (Toko).L2 = 33mH radial choke (Toko).L3 = see Table 1Si = miniature SPST switch.MI = 250 µA moving -coil VU meter.1 off DIN -type 2 -way loudspeaker socket.8 off DIN -type 2 -way loudspeaker plugs.ABS enclosure.

for the resonance meter. The ranges of theresonance meter can only be calibrated ifthe standard choke values listed in Table 1are used.

The circuit diagram shows that thesupply voltage for the resonance meter is18 V, obtained from two series -connected9 V batteries. A mains adaptor with12 VDC output is, however, also suitableif the resonance meter is fitted with a Type78L10 voltage regulator.

ConstructionResonance meters for frequencies up intothe VHF range are not the easiest of con-struction projects. The main problem ofmany home-made as well as ready-mademeters is that these produce 'false' dipswhen the pick-up coil is not held near atuned circuit. According to Murphy's law,these false dips will typically occur in themost frequently used ranges.

886071 - 12

Fig. 4. True -size front -panel layout.ELEKTOR ELECTRONICS SEPTEMBER 1989

RESONANCE METER

The printed -circuit board for the pres-ent resonance meter has been designed tominimize the risk of false dips. Figure 3shows the component mounting plan andthe track layout of the board, which isavailable ready-made.

Mounting the parts on the board is fair-ly straightforward. The only point to payspecial attention to is that all componentwires must be kept as short as possible.

The populated board is mounted in anABS enclosure. This is not standard prac-tice in view of RF screening, but avoids therisk of a metal enclosure affecting theoperation of the oscillator. As a result,false dips would occur, and the calibra-tion would have to be changed.

All wires in the enclosure, and particu-larly those between the board an the in-ductor socket should have the absoluteminimum length. The front -panel designshown in Fig. 4 is copied and secured onto the enclosure.

The pick-up coils are made from DIN -type 2 -way loudspeaker plugs and ready-made chokes as shown in the photographof Fig. 3. The chokes for the two lowestranges must be types with plastic sleev-ing, not types with ferrite encapsulation.The other chokes are miniature axialtypes.

CalibrationThe resonance meter can not be calibratedbefore it has been fitted into a suitableenclosure. Either a frequency meter or ashort-wave receiver must be used for theadjustment procedure.

If a frequency meter is available, theprocedure is started by winding 10 turnsof enamelled copper wire on to a leadpencil. Remove the pencil, and connectthe inductor to the input of the frequencymeter. Plug one of the lower -range coilsinto the resonance meter, switch on theinstrument, and adjust P2 for full-scale de-flection of the signal level meter, Mi. Thefrequency meter will display a frequencyif the pick-up coil on the resonance meteris held near that on the frequency meter.Check whether the displayed frequencyrises if Pi is turned anti -clockwise. If not,swap the outer wires on the poten-tiometer. Set the tuning to the highest fre-quency in the range, and adjust P3 untilthe frequency meter displays the scale fre-quency. Turn Pi to the lowest frequency,and adjust P4 similarly. Once again checkthe upper frequency and correct the set-ting of P3 if necessary.

A short-wave receiver is also suitablefor calibrating the resonance meter, buthas the disadvantage of requiring to bere -tuned for every adjustment.

Since every choke has its particulartolerance, it is necessary to check for scaledeviations in every range of the resonancemeter. If the deviation in a particularrange is unacceptable, try using anotherchoke from another batch but with thesame value indication. Choke tolerance istypically ±20%.

Practical useThe resonance meter is a test instrumentthat becomes easy to operate only grad-ually through regular practical use. Priorto any measurement, the frequency rangemust be determined, and the appropriatepick-up coil selected. In some cases, youwill need to change coils if the resonancefrequency is close to the end of the range.Switch on the instrument, and adjust thesensitivity control, P2, for f.s.d. ( full-scaledeflection) of the signal level meter. Lineup the pick-up coil with the inductor inthe equipment (Fig. 6), and tune carefullyuntil the pointer of the level meter movesto the left. The frequency range is prob-ably fairly large at this stage. To achieve amore accurate dip, move the pick-up coilaway from the inductor while still ensur-ing that they point in the same direction.

Now retune the resonance meter until asharp dip is found.

If the resonance frequency of a tunedcircuit is not known, it is wise to startexamining it in the lowest range of theresonance meter, increasing the rangeuntil a sharp dip is found. This procedureavoids harmonics being mistaken for thenatural frequency.

The resonance meter need not be veryaccurate since its main application is thecoarse dimensioning and adjustment ofinductors, or capacitors that form part ofan L -C tuned circuit - precise adjustmentis invariably done along the lines of thesetting -up procedure with the equipmentturned on. Also, it is useful to note that theresonance frequency of an L -C circuit isgenerally lowered when it is installed inthe circuit, which introduces additionalcapacitance.

Not all tuned circuits can be testedwith the aid of the resonance meter. In-ductors wound on a toroid core, or en-closed by a metal cover, absorb very littleexternally applied energy, and do not pro-duce a dip unless a small external seriesinductor of one or two turns is added tem-porarily. This lowers the resonance fre-quency to some extent, but allows a usefulestimate to be made. Some in -circuit L -Cnetworks will not dip either. Examples arethe heavily damped tuned circuits in theemitter line of a grounded -base transistorcircuit. To measure the resonance fre-quency, either the transistor or the tunedcircuit must be removed.

Finally, the resonance frequency ofseries L -C tuned circuits can not bemeasured unless a capacitor is included inthe circuit that provides a path from theinductor back to the series capacitor. This,in fact, creates a parallel tuned circuit.

ELEKTOR ELECTRONICS SEPTEMBER 1989Fig. 6. Using the resonance meter to 'dip' an L -C tuned circuit.

r-iNITRONICS MONITORA. Rigby

The Centronics interface standard is often said to be withoutproblems in practical use. When a malfunction occurs, however,

many more connections have to be checked than, for instance, on anRS -232 link. The monitor described here alleviates the plight in

debugging a Centronics connection. It is a handy tester thatindicates the levels on all lines simultaneously, including those that

carry pulses.

,arlv all of today's personal computersare equipped with a Centronics port forconnecting a printer. The general accept-ance of the Centronics interface standardhas been helped by the availability ofready-made cables of various lengths, andthe fact that the majority of printer manu-facturers have ensured compliance withthe pinning of the 'blue-ribbon' input con-nector on their products.

Sometimes, however, the installationof a new printer or cable gives rise toawkward problems that take a lot ofprecious time to analyse and resolve. Inthese cases, the present in -line indicatorprovides almost instant fault analysis be-cause it shows the logic state of the data -bits and a number of handshaking andcontrol signals.

Data and handshakingTo ,n,-ure that the computer -to -printerlink works as required, a number of sig-nals must be present, while the use ofothers depends on the equipment used ateither side of the Centronics cable_ At thecomputer side, datalines DO -D7 must be

connected, as v. ell as handshake linesSTROBE, BUS\ and/or ACK (acknow-ledge). Especially the last two signals areprone to cause trouble if the relevant pinsare correctly marked (according to theprinter manual), but not used electrically.

When the Centronics connection isfully functional, the computer puts the bitpattern to be sent to the printer on to theeight datalines, and actuates the STROBEline by pulling it low. This enables theprinter to recognize that the databyte rep-resenting the printable character is stableand therefore valid. Reception of the byteis signalled to the computer by a low -to -high change on the BUSY line. BUSY re-mains high until the printer is ready.Depending on the type of printer, the re-ceived databyte is instantly printed, orstored in an internal buffer memory. Inboth cases, however, the processing (whichis not necessarily the same as printing) ofa character is signalled to the computer bymeans of a high -to -low transition on theACK line.

The processing of received charactersdiffers from printer to printer. Older mod-els print each character immediately after

it has been received, halting the computerduring the printing operation. Printers ofa later generation typically feature a smallbuffer that allows a line of printable char-acters to be stored. The characters in thisbuffer are not printed until a carriage re-turn is received. Many modern matrixprinters have buffers capable of storingmany kilobytes of text, and handle print-ing, data spooling and communicationwith the computer simultaneously. Sometop -range models house more data pro-cessing chips than the average PC com-patible.

Apart form the data and handshakinglines, the Centronics standard specifies anumber of other, auxiliary, functions:

PE Paper Empty Goes high whenthe printer is outof paper.

SEL Select Indicates that theprinter is on lineand ready to re-ceive data.

AUTO Auto Feed Automatic linefeed after a car-riage return.

1NIT Initialize Resets the printerERROR Indicates internal

failure.

The last four lines must be given a fixedlevel, even if they are not used in the ac-tual connection between the computerand the printer. In other words: the mini-mum requirement is that non-connectedactive -low and active -high lines be fittedwith a pull-up and pull -down resistor re-spectively.

The monitorThe Centronics monitor indicates the cur-rent logic level on all lines by means oflight -emitting diodes (LEDs). The databuslines are connected to a Type 74HCT540buffer that supplies sufficient output cur-rent to connect the LEDs direct to ground.A logic high level causes the LED associ-ated with a particular line to light. Each ofthe five status lines is connected direct tothe associated LED. This can be done withimpunity because the signal levels areELEKTOR ELECTRONICS SEPTEMBER 1989

CENTRONICS MONITOR

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Th.7AS

8 " T2O F., A7CNTR

_11_022 9 D7 ACK OA i 1,2..,_40.111(47:1041

law 614" C23 14 e3 Ct e_3,__0,017 R23 BC

0 24G1 G2 PE

0:113 5576 330n 701 Teii.__,

25 19

oidt 624 e_IL-026 SELECT

CCM/mnim _26_0

27028

5VOt\ X1( 5V® 127

291--T ...2L__0

O

36F11011j ci) F118 R63

40MI6 a + IC3 0

R

...1_,,,

K1 IC2 70

IC3b K2555

0

6

R9 917C

s556

011

THR

26

THRES OUT

TA013,TR

T4:::!, tT F

rk5

C2Cj., T 13816 BC c sC' '-ATik PST13BUSY Kai 5576 mom

crop 1730n.11,TcBC557B 6C5478

730n.:.:12.3 II

Fig. 1. Circuit diagram of the Centronics monitor.

fairly steady.Signal lines STROBE, BUSY and ACK

require a different configuration becausethey carry pulses rather than steady le-vels. Three monostables in the form ofType 555 timer chips are therefore used todrive the relevant LEDs. Inverter T3-R1s-Rio ensures that the BUSY LED lightswhen the associated line is actuated (i.e.,logic high). Such an inverter is not re-quired for the ACK and STROBE lines,which are active -low. The associated 555sare housed in dual timer ICs, a 556.

All other signals that may be available,but are not strictly required for correctoperation, are simply passed between therelevant pins of the input and outputsocket of the monitor. Many Centronicscables do not have separate ground wires,but use commoned connector pins at bothends. These pins are often connected by asingle wire.

Sixteen LEDs enable the user of themonitor to locate the possible source oftrouble at a glance: 8 data LEDs, 3 for thehandshaking lines, and 5 for the status

lines.

Power supplyAn external supply will not be required inmost cases because virtually all modernprinters supply +5 V at pin 18, 35 or both.Diodes D4 and Dia ensure compatibility ofthe monitor with these printers, and alsoallow the unit to be powered from an ex-ternal 10 V /50 mA power supply. Regula-tor IC4 then provides the 5 V supplyvoltage for the ICs and LEDs on the board.

ELEKTOR EI.ECTRONICS SEPTEMBER 1989

uoMPUTERs

ConnectionsConnector Kr is a 36 -way Centronicssocket with straight solder pins. Push thesocket on to the PCB edge, while ensuringthat the pins align with the copper islands.Soldering is then straightforward. Con-nector K is removed from a standard Cen-tronics cable plug, and secured as Ki _ Twotypes of connector exist: versions withscrews and versions with clamps for thescreening hood. The screw type is the bet-ter for the present application.

Pin Signal Source

1 STROBE I Computer

2 Data 0 Computer

3 Data 1 Computer

4 Data 2 Computer

5 Data 3 Computer

o Data 4 Computer

7 Data 5 Computer

R rata R r.rimniitpr

9 Data 7 Computer

10 ACK Printer

11 BUSY Printer

12PAPEREMPTY

Printer

13 SELECT Printer

14AUTO ComputerFEED XT

15 n.c.

16 ground

17 cnassis

18 r5 V Printer

19 ground

20 ground

21 ground

22 ground

23 ground

24 ground

25 ground

26 ground

27 ground

28 ground

29 ground

30 ground

31 INIT Computer

32 ERROR Printer

33 n.c.

34 n.c.

35 +5 V Printer

36 n.c.

Ri 0 0 5 8 8 8 8 0 6 6 6,B 8 0 0 00

®z m®cm CD Cal \1® -51CB \\CCI:-

CBCI! \ Ca

:C.

®\"0 0 0

( 0 04) 0 0

0 0 0000

\ 0

Ltk.

1",18

1

K1

1E1

0 0© 0 0)10 rcjcDI

O ie c7) \: c=

O 0 0t17/--\--, ,---,,-.

0

® 0

G

-0; 77, T T,1C U D 0

I91 cc a a a aC2 15. ?5Dll 013T2 T1

D12 '(r 1

a Cr arb. CU 5 (1.5.0 0 0 0

0

a

o

L

DS

C4r00=1051-0 44- ,010

a

T3 T4 IC4

"1° 019C4 _1,4_000412 10 0110 00110

0la

0is OctC 7-0

16:113 I

R1 RB

CB -00110

++0 0

1K2

Fig. 2. T ack layout and component mounting plan. A number of parts must be solderedat both sides of the printed -circuit board.

Parts list

Resistors (±5%):1k0 SIL resistor array

R11;R14;1319-R24= 68052Ra;f112;R17 = 100QRio;Ria;Ria =Ris = 10kRI6= 4k7

Capacitors:Ci ;C4;Ca;C7 = 100nC2;Ca;Cs;Cs = 330n

Semiconductors:= LED; 3 mm; red

Ds;Dio = 1N4148D19 = 1N4001Ti ;T2;1-4 = BC557BT3 = BC547BICI = 74HCT540IC2 = 555IC3= 5561C4= 7805

Miscellaneous:Kt = 36 -way Centronics socket with straightsolder pins.K2 = 36 -way Centronics plug.Enclosure: e.g., OKW model A9407113.PCB Type 890123 (see Readers Servicespage).


AS CROCONTROLby Simon Young*

This article discusses the evolution of the MCS-51 architectureand how to use ASIC technology to extend the set of generic

features contained in the family members.

Intel offers the MCS-51 architecture tocustomers in a number of ways.

The first is via standard products. suchas the 80051BH and 8052. These devicesare designed for the general microcon-troller market, where the internal hardwareresources can be closely matched to thesystem requirements.

The second is via Application SpecificStandard Product AssPs) developed for avertical market sharing a common set ofadditional features. An example is the8005 IFA, which augments the MCS-51core features with a programmable counterarray. an enhanced serial port for multipro-cessor communications and an up/downtimer/counter.

The third way to gain access to theMCS-51 architecture is via Astc. and thisis the subject of this article. Intel offers tocustomers the same capability it uses inhouse to develop ASSPS.

MCS-51 MicrocontrollersThe MCS-5I family of microcontrollerswas designed to meet the needs of embed-ded control applications. The architectureand instruction set were optimized for themovement of data between internal memo-ry and internal peripherals.

Figure la shows the NICS-5 l architec-ture. The Special Function Register (s191)bus connects the internal resources (suchas port latches. timers and peripheral con-trol registers) with the CPU. The128 bytesof on -chip 12,1M (between 00 hex and 7Fhex) can be addressed both with direct(mov data addr) and indirect (Ntov @Ri)addressing modes. Some devices. e.g., the8052. provide an additional 128 bytes ofon -chip RAM for temporary data storagebetween 80 hex and FF hex (dotted inFig. lb). This may be addressed only indi-rectly - forming a useful area for thestack.

The SFR space appears to the CPU as128 bytes of memory located between 80hex and FF hex. This area of memory isaccessed only by direct addressing modes.in order to distinguish it from the addition -

'Simon Young is with Intel Corporation UK)Ltd at Swindon

Typical 80051 core.based design.

al data RANt discussed above. Of the 128locations. 21 are used in the 80051BHstandard product (26 on the 8052).

The 64 Kbytes of external data memo-ry space are accessed with the MOW(instruction.

Another powerful feature of the MCS-51 architecture is the ability to addressindividual bits within certain SFR andinternal RAM locations. All MCS-5 I

devices contain a complete Boolean (sin-gle -bit) processor. The MCS-51 instruc-tion set supports the Boolean processorwith instructions to move, set. clear, com-plement. OR. AND. and conditional branchon bit. This 'bit addressability allowsindividual bits to be tested and modifiedwithout the need of complex maskingoperations, with consequent significantimprovements in speed.


56 COMPONENTS

,intei

FFFF

0000

IVICS-51 MEMORY STRUCTUREPROGRAM DATAMEMORY MEMORY

FF

80

1-.":;ESSNG ADZ.,.EEZ- .43

EA=O EA=1 7F efecr,v.:PUFECT

ADDRESSM00

PSEN

minFig. la

MCS=51 ARCHITECTURE

128 oytes RAM

ExternalInterrupts

CPU

BEZ

TIMER2

TIMEF11

TIMERO

-1 3 PORTS SERIAL PORT

PO P2 P1 P3

Fig. lb

At the time the first members of theMCS-51 family were introduced. therewas no economic packaging for high pin -count devices. The parts were packaged in40 -pin DIL packages: only recently havePLCC packages been used. To provideaccess to the internal hardware resourcesof the device, several functions. includingport input and output signals, externalmultiplexed address/data bus, serial portt/o, external interrupt signals andtimer/counter input signals had to be mul-tiplexed. Clearly. functions multiplexed onthe same pin rnzt not be used concurrent-ly.

TXD RXD

As systems designers developed in-creasingly complex embedded controlapplications, the 8051 required additionalmemory. peripherals and/or i/o ports.These had to be added externally as mem-ory or memory -mapped peripherals.reducing the parallel i/o available on the8051. Fully expanded in this way. only asingle 8 -bit i/o port is available. While theon -chip features and price -performanceratio of the 8051 make it still an attractiveproposition when compared with othersolutions. the end result is different fromwhat the 8051 was designed to be: a sin-gle -chip. stand-alone microcontroller.

UC51: Intel's originalmicrocontroller coreIn 1985, Intel introduced the UC51, whichwas developed from the 1.5 gm CILMOS III

80051BH standard product. The UC51allows designers to integrate the micro -controller core, memory, memory -mappedperipherals and cells from the 1.5 pm stan-dard cell library on to a single chip.

In transforming the 80051BH into theUC51 core cell, the i/o pads and pin multi-plexers were removed. The internalperipherals, multiplexed address -data bus.control signals and input and output portsall have dedicated signals.

With the ability to choose differentamounts of program ROM (zero. 4 K. 8 Kor 16 K bytes) and data RAM (up to l Kbytes) with no loss in functionality, theUC51 has been a very successful part ofInters fisic offering.

In summary. the UC51 provides sys-tems designers the capability to integrate a'fixed' core and memory -mapped periph-erals, complete with user -defined logic. onto a single Asir device. The ASIC resemblesan integrated version of the discrete solu-tion, with increased flexibility because ofthe demultiplexed Vo. However, it is notpossible to apply the full power of thearchitecture and instruction set to memo-ry -mapped peripherals.

UCS51: Intel's next genera-tion microcontroller core

Intel have recently introduced the 1.:CS51family of microcontroller and peripheralcells into the 1.5 p.m CHMOS HI standardcell library. The UCS5 I permits systemsdesigners to connect any of the availableperipheral cells or user -defined logicdirectly into the SFR space. The UCS51cores then access the control registerswithin these peripherals in exactly thesame way as any internal SFR register.

There are great benefits to be gainedfrom directly connecting peripherals to theSFR bus:

instructions operating on peripheralregisters in the SFR space are more code -efficient then accessing memory -mappedregisters indirectly (with movx t. so thatless program memory space is required;

register -direct -instructions tADD. ADDC,

SUBB, LNC, DEC. ANL. ORL. XRL, MOV, PUSH,

POP. XCH, ONE and D1NZ) execute morequickly, giving improved system through-put;

certain bytes in the SFR space (locatedat x0 hex and x8 hex) are bit addressable:mapping peripherals into these locationspermits the bit -banging capabilities of theBoolean processor to be applied to theseregisters:


111ASIC MICROCONTROLLERS

interface logic between the UCS51core and UCS5I peripherals is eliminated:there is no need of an address latch.address decoder or tri-state bus driver.

The basic UCS51 core cell resembles aUC5 1. PORT1 has been removed to provideaccess to the si bus, although it may bereplaced easily as described later.Interfaces have been added for connectingROM modules (either none or one of 4 K. 8K. or 16 K bytes), a RAM module (sameRAM as 8052) and an interrupt expansionunit. A functional cell diagram is shown inFig. 2.

Additional interruptsenhance real-timeperformanceUnexpanded L:CS51 cores have five inter-rupt signals available, as have the UC51and 80C5IBH. Users may configure theinternal peripheral interrupts for use asgeneral purpose interrupt signals. with nochange in priority levels and vector loca-tions. It is also possible, by the use of theInterrupt Expansion Unit, to add a furtherfive external interrupts, making a total of10. with complete flexibility of interruptsource, peripherals, on -chip or off -chiplogic.

Lsi peripherals for configuringunique microcontroller cellsThe Bus Interface Unit is. perhaps. themost important UCS51 peripheral cell.Functionally. it is and 8 -bit input, 8 -bitoutput SFR bus interface. With it, designersmay replace PORT1 and add further demul-tiplexed i/o ports as needed.

This cell is also used to interfacebetween on -chip user -defined logic andthe SFR bus. Thus, customer developedlogic using cells from the 1.5 pm standardcell library may be mapped directly intothe SFR space to gain the advantages dis-cussed previously.

The 8 -bit. 8 -channel successiveapproximation ADC has a nominal conver-sion speed of 20 t.ts at a core frequency of16 MHz. A conversion may be triggeredby hardware or software, with an interruptgenerated on completion.

Timer2 is a 16 -bit timer/counter cell.enhanced over the Timer2 found on the8052 standard product and some ASSPS.

The serial 1/o cell is a full -duplex serialport, enhanced from the serial channelcontained in the 80051BH by the additionof a new mode: Mode 4. This mode pro-vides 9-. 10-. 11- or 12 -bit transfers withvariable baud rate. In Mode 4. theUCS51SIO cell also generates parity fortransmission and detects framing. overrunand parity errors on reception.

S

intei

Demutc eAddress

128 bytes RAM

UCS-51 ARCHITECTURE

EA:err.LInterrupts

TIMER1

T1MERO

Special Function Register Bus

4 10 PORTS SERIAL PORT

TXD RXDPO P2 P1 P3

Fig. 2

The baud rate generator cell is used togenerate clocks for the serial I/o peripheralor for user -defined logic. Operating fromthe 16 MHz system clock, the BRG gener-ates rates from 50 Hz to 4 MHz with anaccuracy better than 02g.

These five LSI peripheral cells and 16distinct core configurations complete theUCS5I offering at the time of launch:more are in development. The 1.5 gmstandard cell library includes ssi. Nisi andt/o functions and may also be inteeratedon a UCS51-based ASIC.

CAD tools aid development ofmicrocontroller-based ASICSThe Design Entry Tool. DEr. provides ahigh-level. menu -driven means of config-uring the core hardware resources. TheUCS51 core options are RAM. ROM orinterrupts. Adding a peripheral requirestwo data to be entered: the peripheral typeand the address of its control registers inthe SFR space.

The DEr outputs a symbol for this core.The desiener simply adds the user -definedlogic he requires, surrounds this with theI/o pads and the design capture is com-plete.

Once captured. the designs netlist istransmitted to MDVS - Intel's MainframeDesign Verification System based onVAX/ZyCad hardware. Full -timing gate -level simulation of the entire chip is possi-ble with vectors written in TDPL - Intel'sTest Pattern Development Language - andExtASM51. ExtASIv151 provides 8051assembly source code and simulation stim-ulus, and synchronizes the execution ofinstructions with external stimulus applied

ASIC LD

to the ASIC.

The simulation output may be viewedas text on the host. or returned to theworkstation for display and review in thegraphics condition.

The ICE-UCS51 In Circuit Emulatorallows the designer to develop and testcode for a CCS51-based ASIC, and to emu-late the completed ASIC (core. peripheralsand user -defined logic) in the target sys-tem. The ICE is a Pc -based emulator sys-tem. offering the same advanced featuresas Intel's other ICE systems.

The UCS51 core is tested with a slight-ly modified version of the 80051BH stan-dard product test program. guaranteeingfunctional and parametric equivalence tothe standard part. The peripherals are test-ed in the same way.

The designer is responsible only for hisuser -defined 102k, and provides TPDL andExtASM51.

The result is standard product!" qualityand reliability: an AQL of 0.1q- is guaran-teed.

SummaryInters family of UCS51 core and peripher-al cells provides the systems designer withunprecedented flexibility in ASIC design.Not only is access provided to the basiccore architecture of the 8051, but also to aspecialized set of peripheral cells. Thedesign tools guide the designer throughdesign capture. simulation and test vectordevelopments. All components of theUCS51 family were developed with oneoverriding aim: to provide guaranteed suc-cess of UCS5I-based ASIC devices.


APPLICATION NOTESThe contents of this article are based on information obtained from

manufacturers in the electronics industry, and do not imply practicalexperience by Elektor Electronics or its consultants.

HAYES-COMPATIBLE V22bis MODEMsource: Silicon Systems, Inc.

The SSI73D2404 is a set of three chips from Silicon Systems Inc. thatprovides the data pump function required to construct a

high-performance 2400bps full -duplex intelligent modem for useover the dial -up telephone network. The chip set includes operatingmodes compatible with CCITT V22bis, V22, V21, as well as Bell 212A

and Bell 103 data communications standards.

Using advanced CMOS processes that in-clude analogue, digital signal processingand switched capacitor techniques, SSI's73D2404 is claimed to offer excellent per-formance and a high level of functionalintegration in a compact three -chip setthat includes one 40 -pin and two 28 -pindual -in -line (DIL) packages.

The chip set is ideal for use in stand-alone as well as integral system modemproducts where full -duplex 2400 bit persecond (bps) data communications overthe two -wire public telephone network isdesired.

OperationThe SSI 73D2-10-1 is a complete V22bis in-telligent modem contained in three CMOSICs. The device set forms the basis for astand-alone modern with self-containedcommand interpreter, indicator LEDs, de-fault switches and interface lines for anRS232 serial port. Both data and com-mands are passed over the serial port ascustomary for modems in the PC environ-ment.

The SSI 73D2404 provides the QAM(quadrature amplitude modulation), PSK(phase shift keying) and FSK (frequency

551 24040S DEMONSTRATOR

shift keying) modulator /demodulatorfunctions, call progress and handshaketone monitors, test modes and a tone gen-erator capable of producing DTMF (dual -tone multi -frequency), answer and CCITTguard tones. This device supports theV22bis, V22, V21 and Bell 212A/103 oper-ating modes, both synchronous and asyn-

SSI MODEM CHIP SET

Multi -mode V22bis. V22, V21 andBell 2121003 compatible device set forintelligent modem designs

Full -duplex operation at 300, 1200. and2400 baud with all operating modes botr.,synchronous and asynchronous

Includes high-level 'AT' command inter-preter compatible with 2400 baud indus-try standard products

Complete complement of 'AT' modemfeatures

Selectable automatic speed select, hang -shake and baudrate detect functions

Supports external non-volatile memoryto store user configurations

Adaptive equalization for optimum perfor-mance over all lines

Dynamic range from 0 to -45 dBm

Call progress. carrier and answer tonedetectors provide intelligent dialling func-tions

DTMF and CCITT guard tone generators

Test modes available: ALB, DL, RDL forcomplete test capability

Space -efficient 28- and 40 -pin DiPs

All-CMOS technology for low -power con-sumption (<600 mW at 5 V)

ELENTOR EI.ECTRONICS SEPTEMBER 1989

HAYES-COMPAT1BLE V22bis MODEM 59

chronous. The 73D2404 is designed to pro-vide functions needed for an intelligentmodem, and includes auto -dial and auto -answer, handshake with auto -fallback,and selectable pulse or DT\IF dialling se-quences to simplify these designs.

The chips set consist of three devices.The SSI73M214 is an analogue processorthat performs the filtering, timing adjust-ment, level detection and modulationfunctions. The 73D215 is the receiver digi-tal signal processor. The 73D216 is a com-mand processor that providessupervisory control and command inter-pretation.

QAMmodulator/demodulatorThe SSI73D2404 scrambles and encodesthe 2400bps incoming data into quad bitsrepresented by 16 possible signal pointsas specified by CCITT recommendationV22bis. The modulator transmits this en-coded data using either a 1200 Hz (origin-ate mode) or 2400 Hz (answer mode)carrier. The demodulator reverses thisprocedure and recovers a data clock fromthe incoming signal. Adaptive equalisa-tion corrects for different line conditionsby automatically changing filter parame-ters to compensate for line characteristics.

PSK modulator/demodulatorIn PSK mode, the 73D2404 modulates the1200bps incoming data using a subset ofthe QAM signal points as specified byCCITT recommendation V22bis, V22 andBell 212A. The PSK demodulator is simi-lar to the QA NI demodulator.

FSK modulator/demodulatorThe FSK transmitter frequency modulatesthe analogue output signal using two dis-crete frequencies to represent the binarydata. The Bell 103 standard frequencies of1270 Hz and 1070 Hz (originate mark andspace) and 2225 Hz and 2025 Hz (answermark and space), or the V21 standard fre-quencies of 980 Hz and 1180 Hz (originatemark and space) and 1650 Hz and1850 Hz (answer mark and space) areused when this mode is selected. De-modulation involves detecting the re-ceived frequencies and decoding theminto the appropriate binary value.

Passband filters andequalisersA bandsplit filter is included to shape theamplitude and phase response of thetransmit signal into a square root, 755'r -raised, cosine, and provided rejection ofout -of -band signals in the receive channel.

Asynchronous modesThe asynchronous mode is used for com-munication between asynchronous termi-ELEKTOR ELECTRONICS SEPTEMBER 1989

SPEED PROTOCOL COMPAT1BLILTY GUIDE

Calling a:

7302404 originating as:

Bell ccrrr300 1200 300 1200 2400

Bet 300 (103) 300 300 300

1200 (212) 300 1200 1200 1200

2400` 1224) 300 1200 1200 2400

CCITT 300 (V.21) 300 - -

1200 (V.22) 300 1200 - 1200 1200

2400 (V.22 his) 300 1200 1200 2400

Called from a:

7302404 answering as:

Bell cc rn-

300 1200 - 300 1200 2400

Bet 300 (103) 300 300 - 300

1200 (212) 300 1200 - 1200 1200

2400 (224) 300 1200 1200 2400

CCITT 300 (V21) - .. 300 - -

1200 (V.22) 300 1203 - 1200 1200

2400 (V.22 his) 3C-3 1203 - 1200 2400

' A Beg 2400 is a V.22 Hs using a 7925 Hz ans-iver. tone wirout uracrarnNed marks

890122 -T1"AT" COMMANDS SUPPORTED(Note. s=string; n.decirnal. 0-255. x.boolean. 01 =I alsefaue)

COMMAND OPT1CMS DEFAULT

A/ Repeats last command the N.A

A Answer NIA

8x BaliCCITT =110 answer tone @1200 (N/A @2400) 1

Ds Mal string specified by s No string

Ex Command echo, 01 = olVon 1

Mn Rack status. WI = orithl N/A

In 113 code. 0/12/3/4 (see Table 8) N/A

Kn SSi test N/A

1.0 Speaker volume. (0)112.3 = kinnecVhi 2

Mn Speaker. 0i1/213 . control (see Table 3) 1

On Orin. Oran on-linairetrainzxi retrain (see Table 4) WA

P Pulse dal Pulse

Or Oulet result, Oil = 1 -quiet 0

R Reverse originate WA

Srokn Set S register (see Table 2) NA

Sn? Return waif) in register n (see Table 2) WA

T Touch tone dial Pulse

Ux User help screen. Sreg, dal string, data format NA

Vx Verbose result, 0/1 otfion 1

xn Result code. 801/2/314 (see Table 1) 4

Yx Enable tong space cisconnect. 1 = enable 0

Z Restore from Non-Volable Memory WA

&Cs Canter detect override, 0/1 = on:non:nal 0

&Dn DM node, 0/12/3 (see Table 5) 0

&F Restore to factory ooh figuration NIA

&Gn[- CCITT guard lone. 0/1/2 = 001800550 0

Six Auxilary relay control 0

MA AsyncrSync rnade. 0/1/2/3 (see Table 6) 0

&Rx Enable RTS/CTS 0

&Sx

&Tn

DSR override. GI .oWnormal 0

Test mode (see Table 7) ftAEN Pulse dal mode. OM .U.SAI.K. 0

&W write current configuration to NVRAM HA

AXn Sync Tx dock mode. 0/1/2-infiext'slave 0

AZs Store a telephone number -string__....

HA

Factory cdrifiguration'81 El Fl t,2 /41 P 00 V1 X4 YO &CO &DO &GO &JO &MO &PO SRO &SO &T4 4X0

Dial string arguments'- delay C = silent answer I = Rash

.. return to command s dal stored nurnbv;r W = meat for tone R.reverse node

'It tne NovRAM nas not been intatzed il may be necessary to Power doer:Power up and typeAT&F&W4cn to property iritiatze modern stale.

890122 - T2

60 APPLICATION NOTES

tID

RED

PITS

CTS

RATE

RATE

RI

PIa A

T2C.X

EXCLPC

OTR

LSO

LETA U. toRI 03

LS35

1.1.58%<131.770

DSO

5 X112WE

RI CO

1.17E

TX

3

LEOTX

/3

<<Cts

R.5 ).133TX/

.1_7X

R233<<

V

LEO

USETX

LEF

LS35

.>2LP1C »Ca

((Hs

9/(2.3-2-COO

R'7

13E

PO21501R

247

LS35

7 to DM

000

A

s vss

CUM 1CCEsvertoi

RTS>>

IMO

GAO

ER

CO

HS

UCFI

ICSO

RE1

Al 2

A2 3 ,1A3Ai 17

I .5"4 7 .5AS

21 1

La .2GNI

cHlCNa U11

Gac.0c20C30c

501.10.c

cSOL

.2*5.1.5

,3YCC

3M9

01271

GAD

A

15.5 R.2 R.3

SPL'»-111F

WA

_L

ram 10

taC

03c

F E

GPO

LP,

XAA14.°SCIfni )1 17to

PART ARED 11=x7 Bi!F..F,,.

00

14:543. -GM

aro

Fig. 1. SSI 73D2404 modem chip set in a sample application circuit.

nals which may vary the data rate from+1.5q to -1.51. When transmitting in thismode. the serial data on the TXD input ispassed through a rate converter that in-serts or deletes stop bits in the serial bitstream in order to supply a signal whosedata rate is accurate to 0.01c -r. The signalis routed to a data scrambler (followingthe CCITT V22bis algorithm) and into the

modulator. The 73D2-10-1 recognizes abreak signal and handles it in accordancewith Bell 212A specifications. Receiveddata is processed in a similar manner ex-cept that the rate converter now acts toreinsert any deleted stop bits. An incom-ing break signal will be passed throughwithout incorrectly inserting a stop bit.

890122-12

Pa

RPt

PA

Synchronous modesSynchronous operation is possible

only with the QAN1 or PSK mode. Oper-ation is similar to that of the asynchronousmode except that data must be syn-chronized to a clock, and no variation indata transfer rate is allowable. Serial inputdata appearing at the ix D terminal must

EI.EKTOR ELECTRONICS SEPTEMBER 1989

HAYES-COMPATIBLE V22bis MODEM 61

Pee

74L533

10

U155 nee

nl- III.J_

Mit" , c rn:_...0CCD 2 39 'ICO

Pa 3 . 30,Ds. i 37 302

2 35 1133Haw 6 z )04

7 at IPSPa MA 3 32 3C

ft67 3 I 3'307

11 i31

331'41"Ta11:713 12 Z13la13 34 AliNME 14 a 1"

11M70 is 33 AU 199ACI3 :31 : iA.M121

32 11 32 3.1*21 2,2 r.9

GM.1 -=OM

IC

0

CU

LAMBEDIMEMc(

wont00DI

0203

aC

DID7

174:0

5

7

9

a

aaaz'

aID IIII IS

IC 17

13 13

14 v3

tomew(

\sour(561415c

f-12 .14-t.=011D

gaffMat

a 3vD02 27 3Paa

POD 3 3314"1)oft 4 a 34c.,s 31UP-1M\ DI C 3 2 2, 173a 'co

raz = 37122.003c i 3, 3TINT5

3r2A

j_ <1c.sda

-anRu.

<=1 Aahz

o 3m=11 utit 17 ]std

dC 33 13 3172ACT

yfg V34 14 Ui 1s 301MID=

.S11110,4..

OPM

Fig. 2. SSI73D2404 system interconnection diagram.

be valid on the falling edge of signalTXCI.K. Receive data at the RXD output isclocked out on the rising edge of signalRXCLK. The async/svnc converter is by-passed when synchronous mode is se-lected and data is transmitted out at thesame rate as is input.

Automatic handshakeThe SSI73D2404 will automatically per-form a complete handshake as defined byV22bis, V22 and Bell 212A/103 standardsto connect with a remote modem. The73D2404 automatically determines thespeed and operating mode and adjusts itsoperation to correspond to that of ananswering modem when originating acall_

Test modesThe SS173D2404 allows the use of anal-ogue loopback, digital loopback andremote digital loopback test modes. Fulltest mode capability allows testing of themodem and interface functions from thelocal terminal using the appropriate con-trol commands, or remotely using theRDL function.

Adaptive equalization withauto -retrainThe SSI73D2404 uses adaptive equaliza-tion which automatically compensates forELEKTOR ELECTRONICS SEPTEMBER 1989

C> 73A

CI zit

arcs

890122-11

varying line characteristics by adjustingtaps on a multi -tap FIR filter. Optimumperformance is obtained with this tech-nique over a wide range of line conditions.When the line quality deteriorates to aspecified level, the 73D2404 can automat-ically initiate a retrain of the equalizer tore-establish data communications with-out the need to go through a completehandshake sequence.

AT -standard commandinterpreterThe SSI73D2404 includes an AT commandinterpreter that is compatible with theHayes 2400 SmartModeniTm command set.Functions and features included with in-telligent modems are provided by the73D2404 command interpreter. An over-view of supported commands is given inTable 1.

Non-volatile memoryThe SSI73D2404 supports connection toan external non-volatile memory, e.g.,Xicor's X2444, to store a dial string and thecurrent AT command configuration. Theuse of a non-volatile RAM is illustrated inthe application circuit.

Source:55173D2404 V.22bis 2400 bps Modem De-vice Set, June 1988, published by Silicon

Systems Inc.

Silicon Systems Inc. 14331 Nlyford Road Tustin CA 92680 U.S.A. Telephone(714) 731-7110.

SSI European Headquarters:Silicon Systems International Woodpec-kers The Common West Chiltington PULBOROUGH RH2O 2PL. Telephone:07983 2331. Fax: 07983 2117.

UK distributor for SSI products is I'rontoElectronic Systems Ltd. City Gate House 399-423 Eastern Avenue Gants Hill Ilford Essex IG2 6LR. Telephone: (01554) 6222. Fax: (01 518) 3222.

62

PRACTICAL FILTER DESIGN PART 8by H. Baggott

The Chebishev section has one of the steepest cut-off profiles of alltypes of filter. Unfortunately, it also has a limiting deficiency: a

ripple in the pass band. The Chebishev filter can be dimensionedin various ways: the ripple is at all times limited to a certain

value. This part of the series includes the Chebishev design tablesfor a ripple of 0.1 dB.

The Chebishev function is one of the mosteffective functions for realizing a filter: itcombines a pronounced bend at the cut-offpoint with a sharp profile. This combina-tion also results in ringing. which. by care-ful design can fortunately be kept within agiven value. There is. however. a directrelation between the cut-off profile and theringing: if the former is made steeper. thelatter becomes more pronounced: and ifthe ringing is kept to a small value. the

Table 10

real part imaginaryn - a part ± 0

2 0.6074 0.7112

3 0.348 0.871

0.696

4 0.2174 0.92920.5248 0.3849

5 0.1466 0.95650.3838 0.59120.4744

6 0.1049 0.97150.2865 0.71120.3913 0.2603

7 0.07846 0.98060.2198 0.78630.3177 0.43640.3526

8 0.06079 0.98640.1731 0.83630.2591 0.55880.3056 0.1962

9 0.04844 0.99050.1395 0.871

0.2137 0.64650.2621 0.3440.2789

10 0.03947 0.99340.1145 0.89620.1784 0.71120.2248 0.45660.2492 0.1573

Table 10. Pole locations ofChebishev filters with 0.1 dBripple.

profile is less steep. In practice. a compro-mise is reached, be -cause in virtually allapplications a ripple exceeding 1 dB isunacceptable. This part and Part 9 willdeal with Chebishev filters with a 0.1 dBand a 0.5 dB ripple respectively. These arethe values that satisfy most applications.

.1 general drawback of Chebishev fil-ters is the very irregular delay time that.for instance, makes the filter unsuitable foruse in loudspeaker cross -over networks.

The computation of the Chebishevpoles can be done in two ways. In the first.use is made of the Chebishev polynomials.while in the second the real part of thepoles of a Butterworth transfer functionare multiplied with a constant factor.which results in a shifting of the polesfrom a circle to an ellipse. Note that in theChebishev polynomials the cut-off point isnot at -3 dB. but the tables take this intoaccount.

Table i11 -C=1--"--- - - - -------iio='-'

0-1=1-----7- -0 I.... ar:.

--'-':'-'-'4÷_0.__-

n C1 L1 j C2 L2 C3 L3 C4 L4 C5 L5

2 0.08908 0.4863 0.228 0.2536 0.2884 0.06999 0.5136 i 0.1539 0.45465 0.2071 0.2476 0.3567 0.2476 0.20716 0.06584 0.4883 0.1524 0.5908 0.1559 0.44467 0.2008 0.2419 0.3564 0.2674 0.3564 0.2419 0.20088 0.06433 0.4779 10.1498 0.5876 0.161 0.5987 0.1554 0.44079 0.1981 0.239 0.3536 0.2678 0.3654 0.2678 0.3536 0.239 0.1981

10 0.06363 0.4729 0.1484 0.5833 0.1606 0.6036 0.1626 0.5995 0.155 0.4388

L1 C1 : L2 C2 L3 C3 L4 C4 L5 C5

Kai t+)

- = 0 0-- -Table 11. Standardized component values for passive low-pass filters with an input impedance to output impedance ratioof 2:1 for even order sections and 1:1 for odd -order filters.

Table 12 -i'---11

0 '-' -ri-i.-.

= Ld

2 ell: 0,4,

n L1 C1 L2 C2 L3 C3 L4 C4 L5 C5

2 0.2214 0.13043 0.2408 0.2402 0.1144 0.2404 0.2814 0.2316 0.1075 0.2485 0.2876 0.2811 0.2256 0.10366 0.2441 0.2998 0.2913 0.2783 0.2218 0.10167 0.2506 0.2957 0.3057 0.2908 0.276 0.2914 0.10048 0.2454 0.3041 0.3025 0.3064 0.2897 0.2742 0.2178 0.09969 0.2515 0.298 0.3117 0.3039 0.306 0.2886 0.273 0.2166 0.09904

10 0.2461 0.3056 0.3058 0.3135 0.304 0.3055 0.2878 0.272 0.2158 0.09864

Table 12 Standardized component values for passive low-pass sections with negligible source impedance.

EI.EKTOR ELECTRONICS SEPTEMBER 1989

PRACTICAL FILTER DESIGN - PART 8 63

Table 13

n C1 C2 C1 I C2 C3

2 0.2607 0.11073 1,0589 0.2905 0.021414 0.7308 0.03836

0.3024 0.19755 1,0838 0.02515

0.7076 0.4011 0.060546 1,5169 0.01767

0.555 0.078260.4063 0.2627

7 2,026 0.013040.7235 0.05301

0.8236 0.5287 0.090618 2,6165 0.01001

0.9188 0.038170.6139 0.10970.3613 0.3697

9 3,285 0.007931,1411 0.028840.7445 0.07409

0.9858 1 0.6622 0.119110 4,0298 0.00643

1,3889 0.022580.8917 0.05340.7078 0.13940.6384 0.4579

Table 13. Standardized component values for active filters with singlefeedback path.

Filter with 0.1 dB rippleTables 10-14 give all information necessary for the computa-tion of Chebishev filters from the 2nd to the 10th order with aripple of 0.1 dB. Note. however, that the values for odd -orderfilters in Table II apply to sections whose input to output impe-dance ratio is 1:2 (if a T configuration) or 2:1 (if a it filter).

The gain vs frequency curves in Fig. 42 clearly show thesharp profile of this type of filter, while the ripple is hardlynoticeable. The delay time vs frequency curves in Fig. 43 givea rather worse picture than those of the filters discussed in pre-vious parts of this article. The step response in Fig. 44 clearlyshows the ringing. Note that the curves in Fig. 43 and Fig. 44do not improve all that much if a lower value ripple is chosen.

Two examplesAs in previous parts. we give two examples of how to computea filter. This time, we take a band-pass filter and a complexlow-pass section in a double opamp configuration.

Example 1.Compute a passive band-pass filter with a centre frequency of1 kHz and a bandwidth of 100 Hz. The attenuation at 900 Hzand 1100 Hz must be at least 20 dB. The output impedance is tobe 600 S2 and the output impedance of the amplifier to whichthe filter is to be connected is negligible.

Solution.Since the centre frequency is known. it need not be computed.We calculate the frequencies corresponding to 900 Hz and1100 Hz but at the opposite side of the filter referred to the cen-tre frequency to find the sharpest roll -off. The frequency asso-ciated with 900 Hz is:

f2= 10002 / 900 = 1111 Hz,

and that associated with 1100 Hz is:


GAIN

1.25

.00

-1.25

-2.50

-3.75

-5.00

I

a=5.10

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1

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a=2

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e=6

v=5

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00eSF1E01ENE1 IN H2

GAIN0815.00

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15.00

5: X10

Fig. 42. Gain vs frequency characteristics of Chebishev filters with a 0.1 dBripple.

DELAYSEC3.15

318.23X5

10316

31-62K5

910

v=5

1003

14

or{

FREOVENCY IN HZ10

Fig. 43. Delay time vs frequency characteristics of Chebishev filters with a0.1 dB ripple.

1.25

1.00

.75

.50

.25

.00

1.25

1.00

.75

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-CO

a-2

n-4

a_6

.-13

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Fig. 44. Step response of Chebishev filters with a 0.1 dB ripple.

64 COMPONENTS

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Fig. 45. Designing a passive band-pass filter: (a)standardized high-pass section: adjusting val-ues for required bandwidth: (c) conversion toband-pass filter

/1=10002 / 1100 = 909 Hz.

The frequencies closest to 1 kHz are909 Hz and 1100 Hz, so that the band-width at the -20 dB points is 191 Hz.

In the characteristics, we now have tofind a filter that provides an attenuation ofnot less than 20 dB at a standardized fre-quency of 191/100 = 1.91 Hz (note thatfor a low-pass section the bandwidth. notthe central frequency, is the basis of thedesign). From the data, we choose a third -order. 0.1 dB Chebishev section: this pro-vides an attenuation of about 22 dB atf= 2 Hz (estimated between a second- anda fourth -order filter).

Figure 45a shows the circuit diagramof a third -order low-pass section. Thestandardized component values arederived from Table 12. Next, the 'realvalues are calculated for a terminalimpedance of 600 fi and a cut-off frequen-cy equal to the -3 dB bandwidth (100 Hz).

= LRIf= 1.4448 H

C1 = Cl/2f= 4.003x 10-6 = 4µF

=LRIf= 0.684 H

Then follows the transformation from alow-pass to a band-pass filter as explainedin Part 5, which results in the filter shownin Fig. 45c. The remaining componentsare calculated with the aid of formulas[34]. and [35] (Part 5):

C-, = 1/LA 2rtfc)2 = 1/1.45(2711000)2 =

R4

E%JcJ- 15

Fig. 46. Example of a state -variable filter with a cut-off frequency of 3 kHz.

L3 = 1/C1( 2402 = 1/4x10-6(2rtl 000)2 =

= 6.33x10-3 = 6.33 mH

C.; = 1/L2(2402 = 1/0.68(27E1000)2.

= 3.73x10-8 = 37.3 nF

Note that the central frequency of theband-pass section is used only for thecomputation of the values of those compo-nents that are added during the conversionprocess.

Example 2.Design an active high-pass filter with acut-off frequency of 3 kHz and a slope of12 dB/octave. It is essential that the cut-off frequency can be set accurately. Thegain of the section must be 14 dB (x5).

Solution.This type of section is best realized by astate -variable filter (see Fig. 17 -Part 3).For convenience. we again choose aChebishev filter. The state -variable filter isbased on the poles of a second -order filterin Table 10:

-CL = 0.6074

=±0.7112

First, we choose a value for C. say.4.7 nF. Resistors R are given a value of33 ka The other resistor values are thencalculated with the aid of formulas [19].[20], [21] and [22]. but note that all valuesso obtained must be divided by the cut-offfrequency since the formulas give valuesforf= 1 Hz.

R1 = 1/[2rtfkACNI(ot24.132)] == 1/27m3000x5x4.7x10-9XKM0.60742+0.71122) = 2414 SI

= 1/47raCfk== 1/4rrx3000x0.6074x4.7x10-9 == 9291

R3 = R4 = 1/2r(Cf01(a2+132)== 1/2rix3000x4.7x10-9xx(0.60742+0.71122) = 12068 Ll

Resistors R-, and R4 may be a combina-tion of a fixed resistor and a preset poten-tiometer to enable the cut-off frequencyand Q -factor of the section to be set accu-rately.

Correction to Part 3From the foregoing in this part of theseries, you will have noticed that resistorsR, and R4 and NOT R1 and R3 (as statedin Part 3) are used for setting the parame-ters of the state -variable section. Thus, R4serves to set the maximum output voltageof Al at to. while R, is used to set thebandwidth to the value at which the Q -fac-tor is calculated.

= 1.75x10-8 = 17.5 nF


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